Improve default window size

Okular's current default window size is very small, and results in more 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 (merged). 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.

Here's how it will look with both this MR and frameworks/kxmlgui!53 (merged):

cc @teams/vdg @aacid @davidhurka