[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
• Use Layouts where appropriate instead of lots of anchors
• Reduce number of unnecessary items
• Fix inappropriate uses mutating width and height inside layouts

Visual changes are extremely minor and of a bugfixy nature.

BUG: 428883 FIXED-IN: 5.21