GUI pannel for prioirty signs

[kianzarrin]

priority signs tool is getting more sophisticated if these features are implemented: #712 #568 .

The problem is the number of modifier keys is getting out of hand. I think too many modifier keys and state machines can confuse users.

One suggestion is to create GUI panel to at least visualize to the user what is going on.
In this branch I created a GUI (for demo only - clear button does not work):

• Multi mode: pressing shift enables entire road/roundabout mode. in this mode all related buttons are blue. the multi mode button uses active sprite. Releasing shift deactivates the multi mode. alternatively pressing the multi mode button once activates it (turning other buttons blue), pressing it again deactivates it.
• Medium Priority mode(quick setup of meduim prioirty road  #712): when pressed it activates. when pressed again it deactivates. CTRL+ALT temporarily activates it.
• High priority Priority mode: when pressed it activates. when pressed again it deactivates. Temporarily activates it.
• stop sign toggle: switches between stop/yield signs
• Clear (Undo/delete feature #568): clears traffic rules (does not work).

see also #557

[kvakvs]

I am soon done with main menu panel and as it is delivered i will look for another UI panel to visit. This can be it. Please don't spend much time making it pretty, instead make it work for all features, while the UI code to remain as simple as possible.

[kianzarrin]

This is just for discussion and I am not saying that we should actually do it. in #681 (comment) @aubergine10 asked to get rid of the vehicle restriction GUI panel.

[kianzarrin]

@kvakvs I am creating this issue only for discussion. Not suggesting that we should actually do it. I did put the effort of creating a demo GUI but I am willing to trash it.