When we first get video, the video component will resize itself. This will
cause the page to be reflowed, but that doesn't trigger an update on the gemini
scrollbar. We therefore need to force an update on the messagepanel. Implement
this by providing an onResize property on the CallView component.
We need to do the same when we change the maxHeight on the video panel.
The same applies to resizing of the MessageComposer. That was previously
handled with a fugly roomView.forceUpdate() from MessageComposer - make it use
the same mechanism.
Finally: the messageComposer is at least 70 pixels, and up to 100 pixels high -
not 36. Fix the auxPanelMaxHeight calculation - and use a static constant
rather than hardcoding the number to avoid this happening again.
