It would be useful to have a presets drop down list below the X and Y position spinboxes which would automatically adjust the X and Y positions according to some useful preset values like
Don't think that page/slide/drawing edges are needed here. Rather including margins. And after all, the lean dialog becomes crowded with all these convenience features.
True not all the edges are needed, so here is a new list that can be expanded.
Vertical Center - useful for vertical snap lines
Center - useful for double lines (bug 112625)
Horizontal Center - useful for horizontal snap lines
The margins are already visible in Draw and objects already snap to it so no need for them.
Another option could be cursor position, which would take the cursor position when the user right-clicked to open the context menu.