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