Messages are marked read even if user is absent

[scoates]

When a user is not at their computer (such as if Mactrix is not the focused app, or if their Mac is in sleep mode), messages are still marked read.

I think we should manage this a little better by at least tracking Mactrix's focus and not marking messages read if it's not focused.

Element's behaviour for this is even a little more complicated, I think (from a user perspective; I haven't looked at the code), where messages aren't marked read right away even if the app is focused; might need some keyboard or mouse interaction.

[viktorstrate]

For future reference, this article on detecting focused window could be useful.