This PR adds support for the `OSC 21` sequence used on DEC terminals to
set the window title. It's just an alias of the `OSC 2` title sequence
used by XTerm.
This PR also corrects the handling of blank title sequences, which are
supposed to reset the title to its default value, but were previously
ignored.
## Detailed Description of the Pull Request / Additional comments
To handle the blank title parsing correctly, I had to make some changes
to the state machine. Previously it would not have dispatched an `OSC`
sequence unless it received a semicolon following the `OSC` number, but
when there's a blank string, that semicolon should not be required.
I also took this opportunity to simplify the `OSC` parsing in the state
machine, and eliminate the `_GetOscTitle` method which no longer served
any purpose.
## Validation Steps Performed
I've manually confirmed that the title sequences are now working as
expected, and added some state machine unit tests covering the blank
value handling for these sequences.
I also had to update one of the existing state machine tests to account
for the changes I made to allow the semicolon to be omitted.
Closes#16783Closes#16784
33589cd8db