Queue ImageLoader requests

[sonicmax]

Currently it attempts to load them all at once and updates UI whenever the request finishes - would be better to load them in order.

[sonicmax]

e059a44

This solves the problem on a per message basis, but still leads to weird behaviour where images from one message will load before images from a previous message. Best way to handle this would probably be to create an ImageQueue class which iterates over each message in order

[sonicmax]

2c9a47b

Now need to figure out how to remove ImageLoaders from queue if message is no longer displayed on screen (to prioritise visible messages when user scrolls).

[sonicmax]

ImageLoaders are aborted/removed from queue once containing view has been detached from window - 8759454

Need to find out what happens after activity is destroyed (ie do ImageLoaders continue to execute?) but I can open a new issue for that.