This commit implements the OSC 9;4 sequence per the [ConEmu style].
| sequence | description |
| ------------ | ------------ |
| `ESC ] 9 ; 4 ; st ; pr ST` | Set progress state on taskbar and tab. |
| | When `st` is: |
| | |
| | `0`: remove progress. |
| | `1`: set progress value to `pr` (number, 0-100). |
| | `2`: set the taskbar to the "Error" state |
| | `3`: set the taskbar to the "Indeterminate" state |
| | `4`: set the taskbar to the "Warning" state |
We've also extended this with:
* st 3: set indeterminate state
* st 4: set paused state
We handle multiple tabs sending the sequence by using the the last focused
control's taskbar state/progress.
Upon receiving the sequence in `TerminalApi`, we send an event that gets caught
by `TerminalPage`. `TerminalPage` then fires another event that gets caught by
`AppHost` and that's where we set the taskbar progress.
Closes#3004
[ConEmu style]: https://conemu.github.io/en/AnsiEscapeCodes.html#ConEmu_specific_OSC
16e8a84cfb