Allow selecting two drivers for persistent comparison charts

[JustAman62]

Closes #126.

In Race sessions, we can now use the C key when the cursor is on a driver to select that driver as part of a comparison. Selecting a second driver then adds a persistent comparison chart to the button of the screen (similar to the existing comparison which applies when cursor is on a driver already).

Usage:

1. Use the cursor to highlight a driver to compare
2. Press C to start the comparison
3. Use the cursor to highlight the second driver you want to compare
4. C to add the current driver at the second part of the compare
5. Comparison charts will now show at the bottom
6. Move the cursor to the top (position 0) to keep show the direct comparison in the Compare tower column as well

One driver selected:

With two drivers selected: