Add a Hotkey class for user-configurable keys. Fixes #3779.

Users can now define and use hotkeys that include modifiers (ctrl/meta/shift/alt).

mods/ra/chrome/settings.yaml

@@ -321,11 +321,10 @@ Background@SETTINGS_MENU:
 									X:10
 									Width:200
 									Height:25
-								TextField@HOTKEY:
+								HotkeyEntry@HOTKEY:
 									X:250
 									Width:139
 									Height:25
-									MaxLength:16
 				Label@KEYS_UNITCOMMANDSHEADLINE:
 					X:0
 					Y:130
@@ -348,11 +347,10 @@ Background@SETTINGS_MENU:
 									X:10
 									Width:200
 									Height:25
-								TextField@HOTKEY:
+								HotkeyEntry@HOTKEY:
 									X:250
 									Width:139
 									Height:25
-									MaxLength:16
 		Container@DEBUG_PANE:
 			X:37
 			Y:100