Fixes for applet configuration layout.

Summary:
Fix applet configuration overlap with top line.
Change the margins to move scrollbar so that it touches the edge.
and use margins for content.
Set content to use available width.
Prevent vertical scrollbar from poping on/off internally.
Change the layout so that categories column can get the full height.
Remove 2 unused properties.
Fix bind loop for pageFlickable's width.
Fix title alignment for RTL.

Test Plan:
Open desktop and audio configurations and make windows small.
Select audio category and move content to the top. (If there is no
scrollbar, increase and decrease height to exploit bug.)

Before:
At the right side (the main) scrollbar will have a margin, and overlap
with content.

Desktop categories stop above buttons cancel, ok.

Main content, and category highlight are displayed on top of the line.

After:
Scrollbar touch the edge, and the content has correct margin.

Categories height is not limited.

Main content , and category highlight(restored, see photo above instead for highlight) touch the line below.

Reviewers: Plasma, VDG, filipf, ngraham

Reviewed By: VDG, filipf, ngraham

Subscribers: ngraham, filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27608