Update GUI without recreating it every time

[narc0tiq]

As somewhat implemented in narc0tiq/prodmon, we should not have to recreate the whole set of GUIEntities every 5 seconds, instead:

• maintain a view model containing all the data that will be shown
• add/remove GUI rows according to the number of rows in the view model
• link the GUI buttons to the appropriate sites by referencing the view model

Most usefully, this helps the GUIEntities remain relatively stable, which allows us to re-raise the GUI update timer from where e7e9e71 left it, and it would finally (hopefully) fix #16 for good.

[narc0tiq]

It might not necessarily end up organized as described, but we should really try to get this in 1.0 (or at least a stretch goal not long thereafter). We could support updating the HUD more often (though it seems we don't really need to) if it wouldn't cause missed clicks every once in a while.