Terminal: Add a "tab busy indicator" to show when a tab has belled or has output

[xmm1989218]

Summary of the new feature/enhancement

if something output to tab which no in view, give it a sign, mean something output in that tab, but you no watch it, has below

green point and blue background mean tab which in your view, green point and white background mean tab no in your view and nothing output in that tab, exclamation mark and white background mean tab no in your view and something output in that tab

Proposed technical implementation details (optional)

[oising]

You mean if something changes in the buffer and that tab isn't focused, you want some kind of indicator? I like the idea.

[xmm1989218]

@oising yes it's very useful feature for me

[DHowett-MSFT]

I've renamed this to make it more discoverable.