Add a new tool, 'gnome-shell-extension-prefs', which can configure extensions

A new tool, 'gnome-shell-extension-prefs' can load a new entry point from
extensions, 'prefs.js', which has an entry point to return a GTK+ widget.
This allows extensions to have their own preferences dialog, without each
extension needing to ship its own Python script and .desktop file.

https://bugzilla.gnome.org/show_bug.cgi?id=668429

data/Makefile.am

@@ -1,5 +1,5 @@
 desktopdir=$(datadir)/applications
-desktop_DATA = gnome-shell.desktop
+desktop_DATA = gnome-shell.desktop gnome-shell-extension-prefs.desktop
 
 # We substitute in bindir so it works as an autostart
 # file when built in a non-system prefix
@@ -69,6 +69,7 @@ shaders_DATA = \
 
 EXTRA_DIST =						\
 	gnome-shell.desktop.in.in			\
+	gnome-shell-extension-prefs.in.in		\
 	$(menu_DATA)					\
 	$(shaders_DATA)					\
 	$(convert_DATA)					\
@@ -76,6 +77,7 @@ EXTRA_DIST =						\
 
 CLEANFILES =						\
 	gnome-shell.desktop.in				\
+	gnome-shell-extension-prefs.in			\
 	$(desktop_DATA)					\
 	$(gsettings_SCHEMAS)				\
 	gschemas.compiled

data/gnome-shell-extension-prefs.desktop.in.in

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+_Name=GNOME Shell Extension Preferences
+_Comment=Configure GNOME Shell Extensions
+Exec=@bindir@/gnome-shell-extension-prefs %u
+X-GNOME-Bugzilla-Bugzilla=GNOME
+X-GNOME-Bugzilla-Product=gnome-shell
+X-GNOME-Bugzilla-Component=extensions
+X-GNOME-Bugzilla-Version=@VERSION@
+Categories=GNOME;GTK;
+OnlyShowIn=GNOME;
+NoDisplay=true