Sync .po files with Transifex and current source code

This required some editing re. number of plural forms. Not exactly sure why
Kazakh still has one plural form there (contacted them for clarification).
Also, Manx (gv), Shan (shn), and Central Kurdish (ckb) languages aren't
supported by Transifex at the moment (contacted as well)...

Remove Belarusian (be@latin) as it was no longer present on Launchpad.
Add Welsh (cy).

.tx/config

@@ -5,3 +5,8 @@ host = https://www.transifex.com
 file_filter = qt/translations/transmission_<lang>.ts
 source_lang = en
 type = QT
+
+[transmissionbt.transmission-gtk]
+file_filter = po/<lang>.po
+source_lang = en
+type = PO

po/CMakeLists.txt

@@ -6,7 +6,6 @@ set(${PROJECT_NAME}_LINGUAS
     ast
     az
     be
-    be@latin
     bg
     bn
     bo
@@ -17,6 +16,7 @@ set(${PROJECT_NAME}_LINGUAS
     ceb
     ckb
     cs
+    cy
     da
     de
     el

po/LINGUAS

@@ -4,7 +4,6 @@ ar
 ast
 az
 be
-be@latin
 bg
 bn
 bo
@@ -15,6 +14,7 @@ ca@valencia
 ceb
 ckb
 cs
+cy
 da
 de
 el

po/shn.po

@@ -15,8 +15,8 @@ msgstr ""
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"
 "Plural-Forms: nplurals=2; plural=(n == 1) ? 0 : 1;\n"
-"X-Launchpad-Export-Date: 2016-09-27 19:08+0000\n"
-"X-Generator: Launchpad (build 18204)\n"
+"X-Launchpad-Export-Date: 2017-01-26 18:49+0000\n"
+"X-Generator: Launchpad (build 18315)\n"
 
 #: ../gtk/actions.c:35
 msgid "Sort by _Activity"