Fix system tray UI/UX & refactor

Summary:
System tray widget had the following UX problem:
Icons are laid out in a Flow QML layout, using minimal amount of
space, thus not filling the height/width of the task bar. In other
words: user can only click directly on an icon, not over or under it.

Consider the following scenario:

Given icon size X and task bar of height 1.5 * X located at the bottom;
User moves pointer down to the limit and tries to click the icon.
Expected outcome: applet is activated.
Actual outcome: nothing happens, because icon (together with mouse
area) floats slightly above the bottom.

Which is inconvenient, especially when most other widgets tend to fill
up the space.

This patch fixes aforementioned problem by refactoring layouts using
modern GridLayout, RowLayout et al., so that icons are arranged in
rows and columns based on their number, and each one fills up its
cell. I also made a handful of minor internal refactorings and fixes.
Unfortunately, due to tight coupling, almost all files needed changes
anyway.

Special note on 'CompactApplet.location': it didn't seem to affect
anything at all, so removed it.

At the end of the day no visual changes should be noticeable. Layout
works in both vertical and horizontal form-factor an all four sides of
the desktop.

Test Plan: Please, check whether 'LayoutMirroring' works properly.

Reviewers: VDG, Plasma, broulik, mart, hein, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: aacid, davidre, davidedmundson, ngraham, ndavis, anthonyfieroni, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19745