Allow to call Display.setRescalingAtRuntime() after shells have been created

[HeikoKlare]

Calling Display.setRescalingAtRuntime() is currently only allowed when no shell has been instantiated yet, as otherwise the behavior is not determined.

The method implementation shall be extended so that it can also be called after shells have been created on the display.

• [ ] When activating rescaling via `Display.setRescalingAtRuntime(true), the shells shall be updated according to their current monitor's scaling.
• [ ] When deactivating rescaling via Display.setRescalingAtRuntime(false), the current scale value shall be fixed, such that the shell behaves as if it was initialized with rescaling at runtime deactivated with the primary monitor having the current scale value of the shell.

[akoch-yatta]

After the investigation regarding #118 I don't know if this is possible, I would event assume it is not possible. Everything that is already created has inherited the DPI Awareness at creation time which cannot be changed afterwards. Only newly created elements will be affected