Cleanup: rename uiBut.dt, uiBlock.dt to 'emboss'

Use 'emboss' instead of 'draw_type' as enum, layout & functions use
the term emboss.

This issue was noted by @Poulpator in D8414, as `dt` is also an
abbreviation for delta-time.

source/blender/editors/interface/interface_intern.h

@@ -233,8 +233,8 @@ struct uiBut {
   const char *disabled_info;
 
   BIFIconID icon;
-  /** drawtype: UI_EMBOSS, UI_EMBOSS_NONE ... etc, copied from the block */
-  char dt;
+  /** emboss: UI_EMBOSS, UI_EMBOSS_NONE ... etc, copied from the #uiBlock.emboss */
+  char emboss;
   /** direction in a pie menu, used for collision detection (RadialDirection) */
   signed char pie_dir;
   /** could be made into a single flag */
@@ -405,8 +405,8 @@ struct uiBlock {
   char direction;
   /** UI_BLOCK_THEME_STYLE_* */
   char theme_style;
-  /** drawtype: UI_EMBOSS, UI_EMBOSS_NONE ... etc, copied to buttons */
-  char dt;
+  /** UI_EMBOSS, UI_EMBOSS_NONE ... etc, copied to #uiBut.emboss */
+  char emboss;
   bool auto_open;
   char _pad[5];
   double auto_open_last;