Replace setUserTitle() with setSessionAttribute()

Summary:
Session:setUserTitle() changes various attributes of the current
session/tab, not just the title; change the name from "Title" to
"sessionAttribute" as that's more representative of what the function
does. Propagate the change where appropriate throughout the code.

Remove Vt102Emulation::requestWindowAttribute(), as it's not used
anywhere.

Update a couple of links to Qt and XTerm documentation, respectively.

Test Plan:
Every thing should work as before, for example changing the window title:

• "Show window title on the titlebar" must be enabled
• unset PROMPT_COMMAND (in case it already sets the window title)
• Change the window title:

printf "\033]2;%s\007" "testing"

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, konsole-devel, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D13147