xyz_grid: allow varying the seed along an axis separate from axis options

[rubberbaron]

Description

• in some scenarios with xyz grid, you might want a set of different images along an axis, but the same one on the other axes. currently, you can either change the seed, or you can use prompt SR to change the prompt. this PR allows you to specify a fixed seed but then vary it along one or more axes so the seed can vary at the same time as the prompt. this may be kinda weird; i use it now and then, but it may not be worth the added UI complexity
• a summary of changes in code:
• • three checkboxes are provided, allowing the seed to vary along each axis independently
• • each selected checkbox effectively causes the seed to increment along that axis
• • if multiple axes are selected, 2D/3D array-indexing computation is performed so all seeds are unique
• an alternative implementation would be to have a switch that just forces varying seeds if there's a prompt SR, but it's possible you might have more than one prompt SR but only want seed variation along one of them; or there might be some other axis option where incrementing seeds is useful, so I went with more general functionality
• if you should choose to accept this PR, please suggest how you would prefer the UI to be laid out and I'll fix it

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

There must be a way to present this better in UI. There also should be a tooltip explaining to user what the option does and what happens without it.

[rubberbaron]

That's why I wrote

• if you should choose to accept this PR, please suggest how you would prefer the UI to be laid out and I'll fix it

The code supports varying along more than one axis at a time, but I don't know if that makes any sense, so if we drop supporting that, I guess it could be a dropdown with 4 choices (none, x, y, z). Other than that I'm out of ideas.