Refactor resizing MUC rosters

Dont trigger resizes on notify::position, because the absolut
handle position changes also on window resizes, but the relative handle
position doesnt change.

Instead we use the notify::position event now to save the new position
to the config file and use left button release event (release of the handle after dragging)
and the window maximize/minimize event to trigger resizes in all controls

This fixes also fixes a issue where a previously hidden roster was shown again
if a new tab was opened.

Fixes #8984