simple "remaining time" display

Short video showing what this is supposed to add:
http://youtu.be/HUEESeWCfAY

Simplifications:
- Assumes full per-second-ratios without the dampening shenanigans the
converter uses internally (thought I would get it more wrong trying to
hook into than ignoring it)
- Since connectedResources is not available in the editor and simulating
staging and fuel lines is a bit too much effort for this simple feature
I only differentiate between NO_FLOW (only resources of current part)
and everything else (all resource amounts in every part on the whole
ship, using TAC-FB or so). I prefer this over not having any display in
the editor, but that’s up to Rover.
- Code tries to find the limit which will be reached soonest. However,
if multiple limits are reached at the same time it just selects one of
those randomly.

One problem: if multiple converter modules are present it’s difficult to
see which constraint info belongs to which converter. No real idea how
to fix that though.

I tried to add/change as few as possible, but it ended up quite a lot.
Especially the add-on seems a bit unnecessary, but I haven’t found a
better way to update modules in editor yet. Feedback welcome!
Oh and I accidentally hit the reformat shortcut once and didn’t notice
it until I was nearly finished. RS happily reformatted some stuff, hope
it’s not too bad…

Rover: please feel free to add/remove/change whatever you want or even
reject the whole pr, hope some parts are useful for you anyway.
added a per-module toggle for the "remaining time" feature
Merge pull request #3 from marce155/master

simple "remaining time" display