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