Separate out a base `MessageContentModel` that can be extended to get the component types from different places. This is used currently for `EventMessageContentModel` but will be used later as part of the rich chat bar.
All display text is now in the text component so it never needs special casing. This also cleans up some of the model parameters so more things come from attributes including location and file data (which was already a qvariantmap anyway).
Also cleaned up the itinerary and file enhancement views,
The video thumbnail is no longer hidden while loading. This makes it
look less buggy (like NeoChat forgot what the thumbnail was) while
loading a large video. To help with contrast, a slight background tint
is added.
