katestatusbar: fix margin in status bar buttons on certain styles
On e.g. the Macintosh style the margin for a QPushButton is way larger than with Breeze or Windows. Therefore to prevent cutting off the text, calculate the margin from the style's PM_FocusFrameVMargin
(currently this is set to 2 on Breeze, and to 4 on the Macintosh style).
Thanks to @waqar for suggesting the fix!