Unstable version

After going through the program again, several bugs have been fixed and
new encountered.

[Fixed]: You now can navigate through text fields
[Fixed]: Text fields now support various additional characters
[Fixed]: After creating a user, the user selection screen updates
correctly.

[Changed]: Added field "visible" in at.jumpandjan.gui.Component for
visibility implementation
[Changed]: Added function isDirty, markDirty and init for
at.jumpandjan.gui.Gui. This change allows update requests.
[Changed]: Added function reloadGuis for at.jumpandjan.JumpAndJan.