Add custom icon for my plugin button in left sidebar

[gustavotrott]

Similarly to #166
It would be useful to support SVG icons in the left sidebar as well!

[GuiLeme]

Hmm, I am feeling that more and more, people are requiring extensible areas that support icon to support custom SVG. If we are adding this feature to the sidebar, we might as well do it for all the others extensible areas, right? Since it's not that hard to implement. What do you think?

[gustavotrott]

Yes, I believe SVG should be the official format for defining icons. Over time, we can phase out the use of BBB-specific icons in plugins, since that "library" doesn’t offer a wide variety of icons and could cause compatibility issues if the client changes in the future.
SVGs are more reliable and much more flexible.

It's open for discussion, but I think the best approach for now is to support SVG across all areas and recommend it as the standard (maybe tweaking all samples to make use of it and no longer use BBB icons).