SGR5/6 blink behaviour switch ("PC style" vs "DEC style").

# Description of the new feature/enhancement

Currently, Terminal seems to handle blink mode (`\e[5` and `\e[6`) in the "DEC" fashion (ie switching the characters to a **faint** mode during their offcycle. I, who grew up with DOS ANSI.SYS and still telnet into ANSI-BBSes woudl very much like to be able to switch to the "PC mode" of completely blanking out the characters during their offcycle.

# Proposed technical implementation details (optional)

Add a setting in **Profiles -> -> Advanced** to switch between:
- DEC Mode: _Characters are rendered in a fainter color when blinking off_
- PC Mode: _Characters are invisible when blinking off_

And, this is just my personal taste, but I do think the "PC Mode" behaviour should be default :D

No files, tests, or entry points are named. Start by locating the profile Advanced settings and the terminal blink rendering logic, then trace how SGR5 and SGR6 are handled. Done means users can select DEC or PC mode per profile and blinking characters follow the selected off-cycle behavior.

Written by the indexing model from the issue text.