[fontinst KCM] Slightly modernize layout

Summary:
The Font Install KCM has a very dated layout. Thisd patch addresses some (but not all) of the aesthetic and usability deficiencies by doing the following:

• Eliminate the tools menu and relocate its two items elsewhere
• Move the GHNS button to the bottom-right corner, like every other KCM
• Put the Scan for Duplicate Fonts... action on the toolbar
• Change Add... into Install from File... to match the style of other KCMs

Test Plan:

Reviewers: Plasma, VDG, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T9954

Differential Revision: https://phabricator.kde.org/D16536