topbar: simplify and generalize a function
topbar: simplify and generalize a function

Rename update_select_btn_sensitivity () to update_select_btn () since
it could perform and other UI changes, and we want name consitency
with other similar UI update functions.

https://bugzilla.gnome.org/show_bug.cgi?id=681089

vbenes/gnome-boxes,vbenes/gnome-boxes