UI: Theme options for checkerboard pattern colors and size

This patch adds ability to set up colors and size of background
(transparency) checkerboard pattern in viewport and 2d editors. No new
backgrounds, only changing colors in existing ones.

This is not the background of the viewport, it is a transparency
checkerboard that is turned on only in render mode, when the
transparency mode is on. And also in 2D-editors, (image, sequencer,
etc).

Reviewed By: Pablo Vazquez, Julian Eisel

Differential Revision: https://developer.blender.org/D6791

source/blender/blenloader/intern/versioning_userdef.c

@@ -216,6 +216,9 @@ static void do_versions_theme(const UserDef *userdef, bTheme *btheme)
    */
   {
     /* Keep this block, even when empty. */
+    FROM_DEFAULT_V4_UCHAR(tui.transparent_checker_primary);
+    FROM_DEFAULT_V4_UCHAR(tui.transparent_checker_secondary);
+    btheme->tui.transparent_checker_size = U_theme_default.tui.transparent_checker_size;
   }
 
 #undef FROM_DEFAULT_V4_UCHAR