Added option to set width/height based on canvas

[levjj]

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.

[sambody]

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).

[levjj]

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.