refactor: DockingDesktop

## functional chnage
- Remove static initialization of DockingUISettings
  and do it in the constructor

## Improve robustness
- Check `currentState` nullity when access
- Avoid always running debug logic
- Change stateChange condition to be currentState != null

## initialize
- Move fields initializer to constructor

## Define with lambda
- Define PropertyChangeListener and ActionListener with lambda

## Optimize loop
- Optimize if-else if-else blocks
- Optimize for-loop

## Style
- Remove redundant initializers
- Optimize empty check with isEmpty()
- clean unused local variable
- javadoc: DockingDesktop
- Simplify DockingDesktop#splitComponent method's stateChange local variable definition
- Replace if block of String comparison to switch block
  in xmlCreateComponent private method