Create a sidebar button component

[IreneStr]

Create a sidebar button component that can be used for the following things:

NB. Please think about a better name than sidebarButton, if possible.

!! Branch from feature/content-analysis-in-sidebar !!

Context of the buttons:

Needs #649

[afercia]

@tim when you have a chance: I was able to copy the "gear" icon from dashicons but I'd need some help for the "dock to the bottom" one 🙂thanks!

[hedgefield]

Here's the icons for the docking icon.