Convert GridLinesMgr to use Qt properties

[treaves]

To help clean up application interaction.

[Bogdan Marinov (daggerstab)]

What do you want to do exactly and why?

[treaves (treaves)]

I'm going to convert objects over to use the Qt event system; I started this a long time back, when Fabien was still active, and it's fallen to the wayside. The issue is that a lot of settings change values manually; they poll other objects to determine what their own state should be. The signals/slot metaphor removes the polling, and eases decoupling in the system. The other thing it helps with is a better understanding in the UI of when connections should be broken before changing values. Several of the GUI classes have values that change, that trigger an event, and that event updates the UI, and that UI update triggers an event, etcetera.

So this is an effort to begin to clean all of that up.