Grid: Color of divisions not visible on some screens

[mity]

(Copied from #54.)

I wanted to ask how to change the color of the divisions, in some screens of laptop and monitors is not well distinguished, it would be nice to be able to darken the color.

[mity]

Currently the grid control does not allow to change the color. Currently it uses GetThemeSysColor(grid->theme_listview, COLOR_3DFACE) to paint it. I believe it is the same what standard listview control uses to paint divisions between columns (when it has style LVS_REPORT and flag LVS_EX_GRIDLINES set via LVM_SETEXTENDEDLISTVIEWSTYLE).

But support for customizing of the color might perhaps be implemented. I will consider it.