Redesign of Multiple-Choice Test

In the new design the QRadioButtons have been replaced with
QPushButtons and the design further specified using QStyleSheets. The
new design contains several improvements:

* Labels numbering the entries indicate which entry is selected by
  which keyboard shortcut.
* Borders around the QPushButton make it very clear which entry has the
  keyboard focus and over which entry the mouse is hovering.
* For correct and wrong answers, the background is changed to green and
  red respectively. The black text on colored background is more
  legible than colored text.

REVIEW: 127305
GUI: Re-design of multiple choice widget. Pictures of the multiple
choice widget are used in the handbook in Chapters "Practice" and
"Grammar Practice Modes".