Only set startingTitle once, clear up title fallback handling (#20214)

This commit ensures that we only set the starting title once when we
open a new terminal pane.

It also consolidates all title selection into Terminal::GetConsoleTitle,
which is now used in the TitleChanged event.

TitleChanged no longer stores a separate copy of the starting title if
an application attempts to _clear_ the title; that seems like a poorer
implementation of what we already had.

This supersedes work in #20204.

Closes #19340
Closes #20204

Co-authored-by: imsh <im.shaedar@gmail.com>