Make history buffer size a configurable option

[ProbonoBonobo]

I've only been using Pick for a few minutes, but I absolutely love it. Huge improvement over gpick. My one gripe is that it only remembers 5 colors at a time, which is really inadequate when trying to create a harmonious color scheme of 8-16 (or more) swatches. I'm sure the stock option is fine for many use cases. Would you consider however making this parameter value configurable by the user?

If not (or in the meantime), would tweaking the max size be as simple as modifying some global variable somewhere, or is it more complicated than that?

[stuartlangridge]

This is a good idea, and I'd like to do it. If you're a Python person, you can work around this by changing one number: the 5 in https://github.com/stuartlangridge/ColourPicker/blob/app/pick/main.py#L733. However, actually adding an option to set this is a bit more fiddly because Pick remembers window size and position, which means that if you set it to 12 or something and then set it back again to 5, you'll still have a window which is big enough to fit 12 and Gtk won't let you resize it back to the previous size. So an actual fix will take some time and thinking, but if you really want it right now you can just poke the source and everything still works in my testing :-)