New: Basic route viewer mode for the main program

[leezer3]

This is a basic implementation of a viewer mode for the main openBVE program, eventually designed to replace Route Viewer.

To Use:

• Set the CurrentOptions.GameMode to developer. (Currently has to be done in-code, no UI option at the minute.

Flaws:

• ~~No event markers.~~
• Game is not paused by default. (Need to figure out the exact controls, so we can essentially just run the sim if people wish to test TOD based effects)
• ~~Requires Route Viewer overlays porting. (Should be a relatively quick job.....)~~

[leezer3]

This now mostly works as-per Route Viewer.

TODO:

• Disable unrelated controls / overlays when in developer mode.
• Camera motion speed is not right when using WASD, check.
• Implement developer mode into the dropdown bit.
• Allow hiding of the overlays
• Merge the main branch, and then specifically disable all hacks when in developer mode. (Developers should find and fix the issues, not rely on workarounds)
• Station jump is up the creak when using the numpad, check this.

[leezer3]

Another PR which has ended up getting out of sync with master....

Have cherry-picked the commit to render the events, and placed that instead behind a new key option (Set to CTRL+SHIFT+E by default)

I think the next step is to cherry-pick the initial loading commit & then rehack the overlays.

[leezer3]

Revisited this today, a little more work done.

Flaws:

• Disabling the overlays is crappy code at the minute.
• Pause needs to be reworked, so that time is paused by default, but we can also change the time, mess with dynamic lighting etc.
• No key etc. overlays
• Viewer mode keyconfig?