Allow user to choose custom gap size for grid alignment

[will-moore]

Fixes #368

Also see https://forum.image.sc/t/spacing-between-images-in-omero-figure/68161

To test:

• select some images to align to grid
• In the drop-down beside the grid button, choose a grid gap size in pixels or choose 'custom' size
• If custom size is chosen, prompt asks for user input of size - only numbers are accepted
• Now, when the align-to-grid button is clicked, the panels will align with the chosen gap between them
• The default option "5% of panel width" is the current behaviour and can be chosen again if desired

[imagesc-bot]

This pull request has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/spacing-between-images-in-omero-figure/68161/4

[pwalczysko]

Tested on merge-ci. LGTM.