UI: Add 'x' icon to text buttons to clear content

This is useful e.g. for search buttons to quickly clear the filter string. We might want to make this optional for python scripts.

source/blender/editors/interface/interface_utils.c

@@ -119,6 +119,7 @@ uiBut *uiDefAutoButR(uiBlock *block, PointerRNA *ptr, PropertyRNA *prop, int ind
 			else
 				but = uiDefButR_prop(block, UI_BTYPE_TEXT, 0, name, x1, y1, x2, y2, ptr, prop, index, 0, 0, -1, -1, NULL);
 
+			UI_but_flag_enable(but, UI_BUT_VALUE_CLEAR); /* might want to make this optional? */
 			if (RNA_property_flag(prop) & PROP_TEXTEDIT_UPDATE) {
 				UI_but_flag_enable(but, UI_BUT_TEXTEDIT_UPDATE);
 			}