feat: allow to define tooltip size

[yohanboniface]

Or do we want a slider to change the font size more gradually ?

fix #1884

[yohanboniface]

Suggestion for improvement when there are many points with tooltips:

• only show markers actually on the map view box
• if there are more than 20 (?) points with tooltips, only render medium and high ones
• if there are more than 100 (?) points, only high are rendered
• if more than 1000 (?) display none