Set minimum window size

Summary:
Kamoso currently doesn't have a minimum window size set so it can be resized into a short vertical line.

This patch sets a minimum window size that matches the 240p webcam resolution (320x240) when the toolbar is subtracted.

Test Plan:
Before:
At its smallest

After:
At its smallest

Reviewers: apol, VDG, Plasma, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D27688