[kcmkwin/kwindecoration] Clean up QML code

Various fixes:
- Remove unneeded PlasmaCore import
- Re-arrange imports to be alphabetical and grouped consistently
- Use standard import names
- Use Kirigami units explicitly
- Remove unused ids
- Remove unneeded cases of setting aproperty to its default value
- Use Layouts where appropriate instead of lots of anchors
- Reduce number of unnecessary items
- Fix inappropriate mutation of `width` and `height` inside layouts

Visual changes are extremely minor and of a bugfixy nature.

BUG: 428883
FIXED-IN: 5.21