[SystemTray] Bunch of fixes and improvements in QML

Summary:
Few unrelated fixes, bunched together:

• move some code from main.qml to PlasmoidItem where it belongs
• easier to understand item size calculation
• where possible, replace parent.parent comparisons with a property in AbstractItem
• get rid of marginHints
• Qt 5.14 compatibility fix in configuration
• formatting etc

Test Plan: Everything should work as before, this is just refactoring.

Reviewers: Plasma: Workspaces, Plasma, davidedmundson, ngraham, broulik

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27085