Rigs-of-rods: Bring back 'help' panel

[CuriousMike56]

Since the full ImGui transition is well underway, I figure now would be the best time to create this.

With the old dashboard (pre-0.39) you could set a 'help' texture to display controls/tips otherwise not shown, for example on the T813 DAKAR:

https://docs.rigsofrods.org/vehicle-creation/fileformat-truck/#help

It would be nice to have this texture displayed again on the vehicle description (CTRL+T) window. Something like this (Obviously with ImGui instead):

[only-a-ptr]

👍 I absolutely intend to do this - precisely this way using the 'description' window - you beat me to present the idea by just few days. 😄

[only-a-ptr]

How's this?