Add visual feedback (color?) for item level

[marob]

It would be nice to be able to differentiate toc levels in the progress bar.

[marob]

One possibility would be to change the style (currently vertical lines) to something different (and better?). For example, some points/circles representing slides with different sizes (or a point surrounded by a circle).

It could be something like:

◉●○○○○●○○◉●○○○○

With ◉ representing level 1, ● level 2 and ○ level 3.