p5: rework event-loop drawing

[sbinet]

right now, p5 relies on Gio to get its re-draw events, together with a little ticker goroutine (that ticks at the requested frequency, requesting Gio for redrawing the canvas).

it's a bit brittle because it's intertwined with other Gio events that will re-draw the canvas (such as a resize of the window), which will confuse users' draw methods (e.g.: the current solar-system draw methods expects a fixed frequency and computes delta-t (and thus positions, speeds and accelerations) from that frequency).

we'd need to somehow separate or distinguish the 2 sets of events. (probably by only re-drawing when the ticker goroutine marked some Proc state as "dirty"?)