Use simple push buttons instead of QCommandLinkButtons

The accessibility of QCommandLinkButton is questionable (in my opinion).
For accessibility tools they appear as radio buttons, but selecting them
immediately causes an action like pushing a button. I think this will
confuse people using screen readers. Using push buttons instead makes
clear that activating the button will have an immediate effect.
Additionally, one can easily navigate with the Tab key between all wigets.
Previously, one had to use a combination of the Tab key (to switch between
the command link buttons and the Cancel button) and the arrows keys (to
switch between the command link buttons).

GnuPG-bug-id: 5832