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