Fixed first character eating

Search "entry_changed" and groups "treeview_changed" callbacks were
calling (indirectly) each other: text changed cleared tree selection, tree
changed cleared text entry.

Changed gpk_application_groups_treeview_changed_cb to clear text
only if selection exists.
Changed gpk_application_text_changed_cb to clear selection only if text
is empty.

https://bugzilla.gnome.org/show_bug.cgi?id=705089