Settings dialog can be opened multiple times

[JayFoxRox]

The settings dialog can be opened multiple times.
That is confusing. Especially because the dialogs aren't synchronized and because of #15 (so the user doesn't know which settings are actually used).

We should only allow to open one settings-dialog.

[Significantbits]

I'm not sure if this should be a separate issue or not, but its related to how the settings dialog is implemented. If you close the main window, the settings dialog stays open. Personally, I feel like that isn't intuitive behavior.