ewmh: Cache idx to avoid xcb_change_property calls
ewmh: Cache idx to avoid xcb_change_property calls

Updates ewmh_update_current_desktop, ewmh_update_number_of_desktops

Airblader/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3-original,i3/i3,i3/i3,Airblader/i3,stapelberg/i3,Airblader/i3-original,stapelberg/i3,i3/i3,Airblader/i3-original,Airblader/i3,stapelberg/i3,stapelberg/i3,i3/i3,Airblader/i3