Added option to set width/height based on canvas

The guide tools can be very helpful for creating layouts in Inkscape. However, it can be tricky to pick the right column width or row height to fill a whole page.

This PR adds an option to the "Add Grid Guides.." extension to set the column width and row height automatically based on the canvas width and height.

Thanks, levjj. Seems like a useful addition, I will look into it. We will need to make it clear that in that case, the column width value will be ignored (pity I can't "gray out" an option depending on other options).

Yeah, it seems it's not possible to disable the column width input. Feel free to change the labels and help texts to make this more clear.