ui: add flamescope

[florianl]

Add a flamescope tab to investigate patterns.

Example output:

[christos68k]

I tested it and it works, but there seems to be a bug (not sure if macOS-specific) where if I try to select a region in the flamescope with click-dragging, I can't do it on the initial click as it won't let me drag the region. I have to click a second time and then I can proceed with dragging.

To reproduce, in the flamescope view:

1. Click "Clear Selection"
2. Initiate a click-drag (you should see that you can't drag the region)
3. Try another click-drag (this time it'll work)

[rogercoll]

Works on Linux as well, code lgtm too. I noticed the same bug that @christos68k describes, just on the first click the selection draws a line. Subsequent clicks works as expected, allowing the user to select a region