Make Text element a sibling of the Rectangle to avoid the text from depending on the rectangle's opacity property. Switch to Nheko theme colors

In the future we should probably add a gradient when clipped...

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.

requires Qt5.12+

I suck at sizing so the images in the message are currently hardcoded to
300 pixels in width...