Keymap: Add Defaults button to revert keymap

Fixes #9980

Since hrev44455 we save your modifier keys settings when you switch
keymaps so you could get your modifiers keys messed up and unable to
get back to normal. This Defaults button allows you to override all
that and go back to the default US-International keymap with default
modifiers so you can start with a fresh slate and set things up the
way you want.

Also moved Revert button to the left side next to the Defaults button.
We do this on other preflets, the right side is reserved for an Apply
or OK button, and, if the preflet doesn't have one, is left empty.

src/preferences/keymap/KeymapWindow.h

@@ -51,6 +51,7 @@ protected:
 			void				_SwitchShortcutKeys();
 
 			void				_UseKeymap();
+			void				_DefaultKeymap();
 			void				_RevertKeymap();
 
 			BMenuField*			_CreateDeadKeyMenuField();
@@ -74,6 +75,7 @@ protected:
 private:
 			BListView*			fSystemListView;
 			BListView*			fUserListView;
+			BButton*			fDefaultsButton;
 			BButton*			fRevertButton;
 			BMenu*				fLayoutMenu;
 			BMenu*				fFontMenu;