add a preferences dialog that could configure termonad during runtime #82

It would be great if Termonad had an options dialog where options could be configured during runtime.

Termonad is modeled off of Roxterm, so I imagine this options dialog would look similar to Roxterm's:

Setting an option in this options dialog should update how all the existing terminals behave.

In order to implement this, you will need to first add a menu option for preferences. That can be done similarly to how it is described in the first step of #10.

Then you will need to make that menu option open a new dialog box as described in step three of #13.

From there you need to show the user a set of options they can change. When they click "OK", you need to actually change the current options in TMConfig in the current TMState, as well as actually applying the updates to the existing terminals.

It might also be a good idea to display a message somewhere telling the user that using the options dialog only lets them set options for the current session. If they want to make their changes permanent, they need to actually set them in the ~/.config/termonad/termonad.hs configuration file.