Improve dialogue size for tablet tester

The tablet tester dialogue had its ‘preferred’ size set to
730 × 385, but this was ignored (for some inexplicable reason),
resulting in a very small painting area (width: 128) for
testing the tablet, and a textedit field that was too narrow
to show a status message without breaking it over several lines.

Now, the minimum width and height are set equal to the preferred
(previously ignored) width and height, and the relative widths
of the tablet tester and the textedit field are modified so
that there’s enough horizontal space for the status messages.
The dialogue has been made slightly wider and less tall.

This also fixes a bug in the tablet tester. When the dialogue
was made very small (so that there were hardly any space for
the tablet tester painting area), Krita would freeze. Now,
it’s not possible to make the dialogue very small, so this
never happens.