Feature/441 "focus on this subtree"

[eh-am]

This PR introduces new functionality called focus on this subtree, based on #441

1 - It allows to change the flamegraph root:

Currently the state is tored locally, but in the future people could use to share a specific subtree.

2 - It still allows zooming

3 - Clicking on the same node undoes the zoom:

4 - You can mix and match zoom/focus, here for example we zoom and then we focus:

This currently is scoped under a contextmenu event, up next this feature would be available under a toolbar as well.

[Rperry2174]

Nice work! Looks awesome so far... I think for the text "Focus on this node" is less clear than "collapse" or "collapse above". Mostly because focus is a little more broad in that it could be interpreted multiple different ways in this context whereas collapse is a specific kind of focus.

[Rperry2174]

This is awesome! Everything seems to be running pretty smoothly and couldn't find any edge cases thanks for adding the double-click functionality too!

[Rperry2174]

Two potential ideas for how we can let people know about the right click functionality:

1. We could put one of these: ⓘ somewhere where when you hover over it it says instructions about how right click adds functionality
2. We could add it to the shortcuts list
   

Up to you do I'd be fine merging this and thinking more about this part later too

[eh-am]

@Rperry2174 Great ideas! Let me merge this and do that work in a separate PR.