TETRAEDGE: Add ScummVM option to hide hotspots

[Thunderforge]

The original Windows version of Syberia was made in the Virtools engine. One of the major differences between that version and the Tetraedge version that ScummVM supports is that hotspots are not visible in the Virtools version (see for instance ​this let's play), but are visible in the Tetraedge versions.

I'd like to request a ScummVM option to hide hotspots. I assume this will be a dropdown with options named something like the following:

• Always show all hotspots (default)
• Show all hotspots on keypress
• Hide all hotspots

The first behavior matches the original Tetraedge behavior, the last option matches the original Virtools behavior, and the middle option would be a new option that reflects what is offered by many games similar to Syberia (e.g. Secret Files).