Enable UI scaling in the scenario and implement Linux menus

Patch from @x-qq
Addresses #195

Note: UI scaling for the scenario editor still needs some work,
but it's basically functional.

src/scenedit/SConscript

@@ -27,6 +27,7 @@ elif str(platform) == "win32":
 elif str(platform) == "posix":
 	scened_sources.extend(Split("""
 		scen.menus.linux.cpp
+		scen.menu.cpp
 	"""))
 
 scened = env.Program("#build/bin/BoE Scenario Editor", scened_sources + common_sources)