Variable number of "grid" crop guides

When cropping an image using "grid" guides, user has no control over the grid size. I thought it would be a nice feature to make grid size variable, and sources confirm that such idea has been considered earlier (iop/clipping.c: "TODO: make the number of lines configurable with a slider?").

Following patch adds "grid cells" slider to "grid" crop guides. Value can now be set anywhere between 2 (lowest value that make sense) and 30 (to allow "thirds" split with high, but reasonable number of cells). Defaults to 9 for backward compatibility.

I've kept the feature of "highlighting" guides that split the view in thirds, if applicable. Additionally, I've added highlights for guides that split the view in halves.