MP Staging: made the faction select button about as prominent as you can get

[ci skip]

Thanks to grzywacz for pointing out the possible use of the unit icon.

(cherry-picked from commit dba3d409fc)

data/gui/widget/button_25x25.cfg

@@ -168,6 +168,14 @@
 	()
 }
 
+[button_definition]
+	id = "settings_mp_staging"
+	description = "MP Staging faction select button."
+
+	{_GUI_RESOLUTION () 60 ("buttons/button_square/button_square_60") ("icons/action/editor-tool-unit_60") ()}
+
+[/button_definition]
+
 # Transparent scrollbar buttons
 
 {_GUI_DEFINITION

data/gui/window/mp_staging.cfg

@@ -66,7 +66,7 @@
 
 						[button]
 							id = "select_leader"
-							definition = "settings"
+							definition = "settings_mp_staging"
 							tooltip = _"Select the faction and leader for this side"
 						[/button]