Fix drawing of buttonText (enforce same left/right spacing)

Fix drawing of buttonText (enforce same left/right spacing)

The computation of the width of the button was wrong. If <--> stands
for TEXT_TO_INSET_OFFSET/2 spacing, and if `[]' marks the button's
limits, then the intent is
  <-->[<-->button text<-->]<-->

Therefore the physical grey rectangle width is
   width - Inset::TEXT_TO_INSET_OFFSET

With this change, the spacing on the right of the button is not larger
than the left one.

Fixes bug #10147.
(cherry picked from commit 516d5d29dce28deab4601ec6aa714ca7ff59e6cd)