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/GHOST_IWindow.h

@@ -331,6 +331,29 @@ public:
 
 	virtual float getNativePixelSize(void) = 0;
 
+#ifdef WITH_INPUT_IME
+	/**
+	 * Enable IME attached to the given window, i.e. allows user-input
+	 * events to be dispatched to the IME.
+	 * \param x Requested x-coordinate of the rectangle
+	 * \param y Requested y-coordinate of the rectangle
+	 * \param w Requested width of the rectangle
+	 * \param h Requested height of the rectangle
+	 * \param complete Whether or not to complete the ongoing composition
+	 * true:  Start a new composition
+	 * false: Move the IME windows to the given position without finishing it.
+	 */
+	virtual void beginIME(
+	        GHOST_TInt32 x, GHOST_TInt32 y,
+	        GHOST_TInt32 w, GHOST_TInt32 h,
+	        int completed) = 0;
+
+	/**
+	 * Disable the IME attached to the given window, i.e. prohibits any user-input
+	 * events from being dispatched to the IME.
+	 */
+	virtual void endIME() = 0;
+#endif /* WITH_INPUT_IME */
 	
 #ifdef WITH_CXX_GUARDEDALLOC
 	MEM_CXX_CLASS_ALLOC_FUNCS("GHOST:GHOST_IWindow")