Also fixes a bug with an empty room settings window introduced in the
port.

Also fixes some resize issues with stupid workarounds to our resize
logic. This really needs to be cleaned up at some point!

This is necessary to support having a picker within QML.
Eventually, this should replace the existing widget-based one.

This turns `paginationInProgress` field of `TimelineModel` into a `Q_PROPERTY`, so the Ui can bind to it.
For the moment, I'm showing the same spinner as we do during initial sync. It's not ideal, on the count of being giant and in the middle but it's better than nothing. We can make it more subtle later.

Also make hidden entries not consume space in Menus with some qml
styles.

fixes #124

 - hides the overlay when prompting for download location
 - cancel re-shows the dialog
 - success closes the overlay
 - would be nice to have a return code from the download fn in
 mtxclient.

Closes #140

This currently assumes the event, that is replied to, is already
fetched. If it isn't, it will render an empty reply. In the future we
should fetch replies before rendering them.