preferences: make gap time configurable

[jeffkaufman]

Instead of using a constant 45min gap, put it in the preferences.

This still needs some work: the change in gap time isn't reflected immediately, only after the next ping fires.

[jeffkaufman]

Is there a better way to do this? I mean, I can just build my own TagTime with the gap hardcoded to something else, but other people would probably like the change too.