Dragable Train signals

[MoraX]

I was thinking about this too lately, and this is the idea I had:

- Shift (for ghost) + some other key while holding a signal indicates you want to place intermittent signals
- The default length is the train length you have defined
- You can alter this length by scrolling, each scroll tick adds or removes 1 train car length
- When placing, it places signal ghosts along the entire rail block (left and right of your position), based on the block where you place the signal under your cursor.