Feature request: add API to add custom icons and functionality behind it to left bar.

[Webtakeme]

Currently on left side bar VSCode have only 5 icons (explorer, search, git, debug, extensions).
Below this icons there is really huge empty area.

It would we great if we will have some API to add custom icons to this area and assign some command to it (as I know there is such possibility but only for bottom bar).

Some action require couple step to perform it (run some command):

• open command palette
• search some command and perform it
• if in current command there is subcommands we need to perform couple command selection...

We can avoid this buy adding special icon with assigned action to left bar - only on click to perform some command (it will save us a lot of time during development).

For example on Atom there is extension "tool-bar" https://atom.io/packages/tool-bar that implement such functionality:

Huge empty area:

[bpasero]

#710