Added GDK_KEY_Alt_L/R to the modifier key handling in GTK+ key events.
Added GDK_KEY_Alt_L/R to the modifier key handling in GTK+ key events.

hajimehoshi/ui,UIKit0/ui,UIKit0/ui,mirrr/ui,robbiev/ui,292388900/ui,UIKit0/ui,robbiev/ui,janstk/ui,hajimehoshi/ui,beni55/ui,ProtonMail/ui,ProtonMail/ui,beni55/ui,292388900/ui,beni55/ui,cainiaocome/ui,cainiaocome/ui,ProtonMail/ui,HunterChen/ui,mirrr/ui,janstk/ui,HunterChen/ui,janstk/ui,robbiev/ui,292388900/ui,hajimehoshi/ui,HunterChen/ui,cainiaocome/ui,mirrr/ui