Input Method Editor (IME) support for text buttons

Original patch by @random (D765) with some minor work done by @campbell
and me.

At this place, I'd like call out a number of people who were involved and
deserve a big "Thank you!":
* At the first place @randon who developed and submitted the patch
* The Blendercn community which helped a lot with testing - espacially
* @yuzukyo, @leon_cheung and @kjym3
* @campbellbarton, @mont29 and @sergey for their help and advises during
* review
* @ton who realized the importance of this early on and asked me for
* reviewing

We are still not finished, as this is only the first part of the
implementaion, but there's more to come!

intern/ghost/SConscript

@@ -154,6 +154,13 @@ if env['BF_GHOST_DEBUG']:
 else:
     sources.remove('intern' + os.sep + 'GHOST_EventPrinter.cpp')
 
+if env['WITH_BF_IME']:
+    if window_system in ('win32-vc', 'win32-mingw', 'win64-vc', 'win64-mingw'):
+        defs.append('WITH_INPUT_IME')
+    else:
+        sources.remove('intern' + os.sep + 'GHOST_ImeWin32.h')
+        sources.remove('intern' + os.sep + 'GHOST_ImeWin32.cpp')
+
 if env['WITH_BF_3DMOUSE']:
     defs.append('WITH_INPUT_NDOF')