Move message icons out of the tileset

[lmoureaux]

Is your feature request related to a problem? Please describe.
Currently, tilesets define icons for game events. There is a single set of such icons; they are tiny (8x16) and do not work well over a dark background.

Describe the solution you'd like
Make these icons part of the theme instead. Bump them to SVG rendered at 16x16 or even 20x20, with automatic light and dark variants.

Describe alternatives you've considered
Improve the current icons in the tileset. Cannot have multiple colors.

Additional context
New events being added in #2359