desktopappinfo: add Exec to searchable keys

Add the basename from the first component of the Exec line to the list of
strings to search for via g_desktop_app_info_search().

We treat Exec as a fairly strong match -- just below the visible name.

Add a testcase to make sure everything is working OK.

https://bugzilla.gnome.org/show_bug.cgi?id=725023

gio/tests/desktop-files/usr/applications/eog.desktop

@@ -21,7 +21,7 @@ Comment[en_GB]=Browse and rotate images
 Comment[eo]=Foliumi kaj turni bildojn
 Comment[pt]=Navegue e rode imagens
 Comment[pt_BR]=Navegue e gire imagens
-Exec=true %U
+Exec=false --nonsearchable %U
 Icon=eog
 StartupNotify=true
 Terminal=false