[Applet] Make spinboxes editable

Summary: Otherwise you can't enter numbers in them and it's really annoying

Test Plan:
Now you can change the max volume and volume step spinboxes without raising your
blood pressure

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24816