Force-Ungrab Mouse in Error Dialog

This is probably only/mostly affecting Linux (Here Kubuntu) because of how it handles mouse ungrabbing (you are unable to see a cursor unless you kill the process):
When an exception occurs, SimpleApplication has an ExceptionHandler which spawns an AWT dialog with a partial error message and an OK Button.
Since the mouse is not ungrabbed though, you have to tab until you reach the Button and then press return or space.
So it would be wise to add a call to inputManager.setCursorEnabled(false); in the Exception Handler for these cases (and maybe while we're at it adding a stacktrace output to this dialog)

We just have to find out if there are other uses of https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-desktop/src/main/java/com/jme3/system/JmeDesktopSystem.java#L119 were it would be bad to disattach the cursor automatically

Edit: Maybe we could/should also put this code here instead: https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/app/LegacyApplication.java#L615

Start with the ExceptionHandler in SimpleApplication and inspect the referenced locations in jme3-desktop/src/main/java/com/jme3/system/JmeDesktopSystem.java around line 119 and jme3-core/src/main/java/com/jme3/app/LegacyApplication.java around line 615. Compare their cursor-handling behavior and determine where the error dialog should release the mouse. Done means the dialog can be operated normally when an exception occurs without breaking other cursor uses.