fixes #1661

Fixes #1125

A new window could have the same `QWindow *` value as an already free'ed window,
so using a `QSet<QWindow *>` with potentially free'ed windows might not be reliable.
Use dynamic properties instead.

Co-authored-by: DeepBlueV7.X <nicolas.werner@hotmail.de>

(And shrug losing arms in replies)

More consistent language with similar description strings
Add manpage entry
Add /unignore
Add ignore & decline button in the invite page
Add invite rejection to all ignore methods

The event seems to be very expensive on certain platform theme plugins.

Fixes #1639

See https://bugreports.qt.io/browse/QTBUG-110446

This is a bit hacky, because we open a window to actually get a valid
activation token, but...

Before we only did that, when we basically didn't have the key yet. But
since we usually get sent a RoomKey when a new message is sent after we
sign in, we were discarding, that those messages should usually now be
trusted.