Drivers UI Cleanup (Part of T55145)

* Remove "Show Debug Info" option. Everyone has it turned on all the time,
  since it's just useful to have
* Make the "Remove Driver" button less prominent. It doesn't happen that much,
  so it shouldn't take up as much room
* Make "expressions" textbox wider (i.e. taking up the whole column width)
  by separating the label and textbox on separate lines.
* Rename "Add Variable" button to make it clearer to users what "variables"
  may be (i.e. they serve as a way to specify Inputs, just like adding a
  "Source Data" node in a nodetree)
* Regroup buttons

source/blender/editors/animation/drivers.c

@@ -117,7 +117,6 @@ FCurve *verify_driver_fcurve(ID *id, const char rna_path[], const int array_inde
 			
 			/* add some new driver data */
 			fcu->driver = MEM_callocN(sizeof(ChannelDriver), "ChannelDriver");
-			fcu->driver->flag |= DRIVER_FLAG_SHOWDEBUG;
 			
 			/* F-Modifier or Keyframes? */
 			// FIXME: replace these magic numbers with defines