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.