Improve default window size

Okular's current default window size is very small, due to a bug which
will be fixed by
frameworks/kxmlgui!53. Once that
is fixed, its default window size will become huge due to the current
sizeHint value, which basically says "take up 75% of the screen space".
This may be somewhat excessive.

This commit reduces the sizeHint to a saner default size while still
being big enough to show all toolbar buttons.