Add support for Set Cursor Style (DECSCUSR) escape sequences

Summary:
This follows the XTerm escape sequences, which include a couple more
shapes than what VT520 supports.

BUG: 347323
FIXED-IN: 18.08

Test Plan:
To test the escape sequences:
printf '\e[ q' #blinking block
printf '\e[0 q' # blinking block
printf '\e[1 q' # blinking block
printf '\e[2 q' # steady block
printf '\e[3 q' # blinking underscore
printf '\e[4 q' # steady underscore
printf '\e[5 q' # blinking ibeam/bar
printf '\e[6 q' # steady ibeam/bar

Executing reset in terminal will reset the cursor shape and blinking
settings to the current profile values.

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D12193