Research mouse-locking options on Linux

[Cervator]

Follow-up to #360 which upgraded DS to use LWJGL3 on the backend. As noted by @BenjaminAmos in that PR "Mouse locking has been disabled on Linux, as it does not appear to work anymore (the mouse co-ordinates returned are incorrect when the cursor is locked, so the mouse cursor is never visible when locked)."

See SolInputManager.java for the related code. Probably this is a subject that can be researched decently online for an available alternative. Alternatively if we don't need it / can find a whole other approach then suggestions are welcomed!

May also want to look through older closed issues or PRs that may have touched the area in the past.

[Avalancs]

First interesting idea on SO I found uses java.awt for this:

1. you can query the actual mouse position with https://docs.oracle.com/javase/8/docs/api/java/awt/PointerInfo.html#getLocation--
2. you can reset the mouse to the center with https://docs.oracle.com/javase/7/docs/api/java/awt/Robot.html#mouseMove(int,%20int) I'll try it out when I have some time