Apply all the required styling to show links, table, spoilers, etc in cpp. This also updates the method of revealing spoilers so now you can click to reveal then click again to hide.
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,
And optionally show an error message, if libQuotient gets one. This
helps in certain rare situations where you just *Cannot* download
something. Before, NeoChat would just reset the image or video without
telling you anything but now there's a nice big error message telling
you what's wrong.
