This patch fixes numerous bugs in KoResourceItemChooser

1) Resizing the widged and then hide/show used to reset currently
   selected resource. That caused user's paintop presets and styles
   being invalidated by simply showing the widget on screen. Now
   this problem has gone, because the madoel()->reset() call is
   done in two stages with first saving the current selection and
   then restoring it when the reset is finished.

2) Now the number of columns is calculated dynamically when the widget
   is being resized. Before the patch there was a weird behaviour:
   when resizing the widget the tiles were scaled only, and after
   the next hide/show the number of columns would be recalculated.
   Now both steps are executed simultaneously.

3) The size of the preview splitter in KisPatternChooser is now
   fixed, so the user would see at least something :)

REVIEW: 123486