Require GTK+ 3.13.2

For automatic RTL icon support

https://bugzilla.gnome.org/show_bug.cgi?id=734911
Remove the LED array

Use left and right buttons in the header bar instead. The LED was
reported to be too confusing (it's not clearly a level) and also too
difficult to read compared to normal font.

https://bugzilla.gnome.org/show_bug.cgi?id=734911