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!

source/blender/editors/interface/interface_draw.c

@@ -395,6 +395,12 @@ void UI_draw_roundbox(float minx, float miny, float maxx, float maxy, float rad)
 	ui_draw_anti_roundbox(GL_POLYGON, minx, miny, maxx, maxy, rad, roundboxtype & UI_RB_ALPHA);
 }
 
+void UI_draw_text_underline(int pos_x, int pos_y, int len, int height)
+{
+	int ofs_y = 4 * U.pixelsize;
+	glRecti(pos_x, pos_y - ofs_y, pos_x + len, pos_y - ofs_y + (height * U.pixelsize));
+}
+
 /* ************** SPECIAL BUTTON DRAWING FUNCTIONS ************* */
 
 void ui_draw_but_IMAGE(ARegion *UNUSED(ar), uiBut *but, uiWidgetColors *UNUSED(wcol), const rcti *rect)