WebXDC improvements

* WebXDC "store" to choose a widget to add
* WebXDC UX improvements, especially for widgets that use touch events
* WebXDC experimental "realtime" API support
* Improve display of images shared from WebXDC
* Fix top spacing in chat bubbles
* Fix colour of "send private message" indicator
* Improve thread identicon contrast
* Fixes for rich reply markup processing
* Fix for large font on a retracted message that was a reply
* Improvements to showing what a MUC "actually" sent