kcms/mouse: Code quality improvements

Sort out unused imports and import aliases in QML

Strip superfluous whitespace from a button text label

Fix indentation

Avoid static downcast in backends

Ideally, there should be a common interface for devices instead of a generic QObject with completely custom sets of properties in backend implementations.

Improve typing and qualified property access

Set standard spacing on layouts

Explicitly specify arguments in signal handlers

Injection of parameters into signal handlers is deprecated. Use JavaScript functions with formal parameters instead.

Port to pragma ComponentBehavior: Bound