Guake 3 support

Guake-indicator now supports Guake3 virtual terminal.
Guake3 is a GTK3 rewrite of Guake 0.X, to start guake-indicator for guake3 add the "-guake3" switch from the command line.
For example
cd guake-indicator
./src/guake-indicator -guake3

Guake indicator still relies on GTK2.