configurable GridItem tick spacing and pen color

this patch adds configuration options for GridItem grid and text color, and options for custom grid line spacing.

It also makes GridItem honor the 'foreground' config option.

this version uses setTextColor as advised. It also fixes setting of just one of the x/y tick spacings, and puts setting of the text color in the proper place.
Need to set `self.picture = None` before `update()` (also in the other set methods).