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