[Applet] Fix empty space and give correct size
Summary:
Make default size same with system tray. Inside area is bigger,
without the system tray heading.
Fix empty space.
Test Plan:
Use applet outside the system tray on taskbar or desktop with
display scaling.
Bug: 393418
Bug: 413448
Reviewers: VDG, Plasma, drosca, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26548