Make preferences dialog non-modal (#4391)

This regressed with the switch to `Gtk::Builder` because the dialog was
non-modal before.

This is the cause of the issue with blocklist update status dialog: it
seems that if there's a modal dialog running, showing another non-modal
dialog will make it inaccessible to the user until the first dialog is
closed thus leaving the modal mode.

gtk/ui/gtk3/PrefsDialog.ui

@@ -7,7 +7,7 @@
     <property name="border-width">6</property>
     <property name="title" translatable="yes">Transmission Preferences</property>
     <property name="role">transmission-preferences-dialog</property>
-    <property name="modal">True</property>
+    <property name="modal">False</property>
     <property name="type-hint">dialog</property>
     <child internal-child="vbox">
       <object class="GtkBox" id="dialog_layout">

gtk/ui/gtk4/PrefsDialog.ui

@@ -3,7 +3,7 @@
   <requires lib="gtk" version="4.0"/>
   <object class="GtkDialog" id="PrefsDialog">
     <property name="title" translatable="1">Transmission Preferences</property>
-    <property name="modal">1</property>
+    <property name="modal">0</property>
     <child internal-child="content_area">
       <object class="GtkBox" id="dialog_layout">
         <property name="orientation">vertical</property>