Enlarge default window size. Use (adjusted) calculation in `SizeHint` to determine minimum window width.

Summary:
Call `setMinimumSize()` at each run to enforce a minimum size. Any enlarged size will be saved and restored via the customary method of saving window geometry.

BUG: 389617

Test Plan:
- Compile systemsettings with patch
- Launch system settings (systemsettings5)
- Determine that window opens at an appropriate size.
- Close System Settings and reopen; ensure size remains set

To test small or rotated screen sizes:
- Adjust display resolution to a size smaller than the standard target of 1024x700
- System settings window should launch with a margin on all sides (not monopolize entire screen space)

Reviewers: ngraham, mart, davidedmundson, hein, #plasma

Reviewed By: ngraham, davidedmundson, hein, #plasma

Subscribers: davidedmundson, cfeck, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12252