gtk: add AppData file

This file was originally written by Richard Hughes, for Fedora, who
proposed it on the Transmission forum here:
https://forum.transmissionbt.com/viewtopic.php?f=3&t=16443

It's used to display a nice description and screenshot in GNOME
Software. The format is documented here:
https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html

The <_name/> and <_summary/> fields match the Name= and Comment= fields
in the .desktop file so are already translated.

The screenshot is taken with Transmission 2.92 (14714) on Fedora 25.
It's at one of the sizes recommended by the GNOME Shell Screenshot
Window Sizer extension.

The <update_contact/> is based on the Git history.

gtk/CMakeLists.txt

@@ -50,6 +50,7 @@ add_custom_command(
 
 if(ENABLE_NLS)
     find_program(INTLTOOL_MERGE_EXECUTABLE intltool-merge REQUIRED)
+
     set(${PROJECT_NAME}_DESKTOP_FILE "${PROJECT_BINARY_DIR}/${TR_NAME}-gtk.desktop")
     add_custom_command(
         OUTPUT ${${PROJECT_NAME}_DESKTOP_FILE}
@@ -57,6 +58,14 @@ if(ENABLE_NLS)
         DEPENDS ${PROJECT_SOURCE_DIR}/transmission-gtk.desktop.in
         VERBATIM
     )
+
+    set(${PROJECT_NAME}_APPDATA_FILE "${PROJECT_BINARY_DIR}/${TR_NAME}-gtk.appdata.xml")
+    add_custom_command(
+        OUTPUT ${${PROJECT_NAME}_APPDATA_FILE}
+        COMMAND ${INTLTOOL_MERGE_EXECUTABLE} --xml-style --utf8 ${CMAKE_SOURCE_DIR}/po ${PROJECT_SOURCE_DIR}/transmission-gtk.appdata.xml.in ${${PROJECT_NAME}_APPDATA_FILE}
+        DEPENDS ${PROJECT_SOURCE_DIR}/transmission-gtk.appdata.xml.in
+        VERBATIM
+    )
 endif()
 
 set(${PROJECT_NAME}_SOURCES
@@ -148,6 +157,7 @@ add_executable(${TR_NAME}-gtk WIN32
     ${${PROJECT_NAME}_SOURCES}
     ${${PROJECT_NAME}_HEADERS}
     ${${PROJECT_NAME}_DESKTOP_FILE}
+    ${${PROJECT_NAME}_APPDATA_FILE}
     ${${PROJECT_NAME}_WIN32_RC_FILE}
 )
 
@@ -188,6 +198,7 @@ endif()
 
 if(ENABLE_NLS)
     install(FILES ${${PROJECT_NAME}_DESKTOP_FILE} DESTINATION ${CMAKE_INSTALL_DATADIR}/applications)
+    install(FILES ${${PROJECT_NAME}_APPDATA_FILE} DESTINATION ${CMAKE_INSTALL_DATADIR}/appdata)
 else()
     install(FILES transmission-gtk.desktop.in DESTINATION ${CMAKE_INSTALL_DATADIR}/applications RENAME ${TR_NAME}-gtk.desktop)
 endif()

gtk/Makefile.am

@@ -100,6 +100,13 @@ DESKTOP_FILES=$(DESKTOP_IN_FILES:.desktop.in=.desktop)
 Productivitydir = $(datadir)/applications
 Productivity_DATA = $(DESKTOP_FILES)
 
+APPDATA_IN_FILES=transmission-gtk.appdata.xml.in
+APPDATA_FILES=$(APPDATA_IN_FILES:.xml.in=.xml)
+@INTLTOOL_XML_RULE@
+
+appdatadir = $(datadir)/appdata
+appdata_DATA = $(APPDATA_FILES)
+
 icon_DATA = transmission.png
 icondir = $(datadir)/pixmaps
 

gtk/transmission-gtk.appdata.xml.in

@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+Copyright 2014 Richard Hughes <richard@hughsie.com>
+Copyright 2017 Endless Mobile, Inc.
+ -->
+<component type="desktop-application">
+  <id type="desktop">transmission-gtk.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-2.0 OR GPL-3.0</project_license>
+
+  <_name>Transmission</_name>
+  <_summary>Download and share files over BitTorrent</_summary>
+
+  <description>
+    <!-- Translators: these are the application description paragraphs in the AppData file. -->
+    <_p>
+      BitTorrent is a peer-to-peer file-sharing protocol that is commonly used to
+      distribute large amounts of data between multiple users.
+    </_p>
+    <_p>
+      Transmission is a BitTorrent client with an easy-to-use frontend on top a
+      cross-platform backend.
+      Native frontends are available for OS X and Windows, as well as command line and
+      web frontends.
+    </_p>
+    <_p>
+      Notable features of Transmission include support for Local Peer Discovery, encryption,
+      DHT, µTP, PEX and Magnet Link.
+    </_p>
+  </description>
+  <url type="homepage">https://transmissionbt.com/</url>
+  <screenshots>
+    <screenshot type="default">
+      <image type="source">https://raw.githubusercontent.com/transmission/transmission/master/gtk/screenshots/a.png</image>
+    </screenshot>
+  </screenshots>
+  <update_contact>info_AT_transmissionbt.com</update_contact>
+</component>

po/POTFILES.in

@@ -15,6 +15,7 @@ gtk/open-dialog.c
 gtk/relocate.c
 gtk/stats.c
 gtk/torrent-cell-renderer.c
+gtk/transmission-gtk.appdata.xml.in
 gtk/transmission-gtk.desktop.in
 gtk/tr-core.c
 gtk/tr-icon.c