Port general settings and save setting to ui files

Summary:
Next step after porting the settings to KConfigXT. I recreated the layout as it
was before. Additionaly the KUrlRequester for the default save location now
only accepts folders.

Test Plan: Works as before

Reviewers: #spectacle, ervin, ngraham

Reviewed By: #spectacle, ervin, ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D27373