Gtk UI: Disable all widgets when device sync is off

In the preferences dialog, disable all widgets in the
"Device sync" tab if device sync is not enabled.
Gtk UI: Show error message for unconfigured devices
Gtk UI: Don't close preferences when WebKit/Gtk is missing

When trying to login to Flattr without WebKit/Gtk installed,
the preferences dialog should stay open after the message box.
Gtk UI: Resizable shownotes view in main window
Gtk UI: Searching in extensions list, keyboard interaction

Allow search-as-you-type searching in the extensions list.

Use separate columns in extensions list to allow interaction
with keyboard (enable/disable and show extension infos).

Remove custom mouse button handling, which was confusing.
Gtk UI: Shownotes text view must not be editable
QML UI: Small triangle new indicator in podcast item
QML UI: Fix top margin of input sheet
Updated translation templates from source
Updated Czech (Czech Republic) translation
Updated German translation
Updated Spanish (Spain) translation
Updated Polish translation
Updated Portuguese translation
Updated Portuguese (Brazil) translation
Util: Fix unit test (use real unknown extension)
README: Add dependency information for Sailfish Silica
Update copyright years for 2013
gPodder 3.5.0 "The After Hours" released
Enqueue episodes in Totem also (bug 1775)

Thanks to Eric Le Lay for the patch