Unify BrainGlobe Modal dialogs

[alessandrofelder]

BrainGlobe widgets should all have the same look-and-feel and we should reuse code where possible. We currently have two different ways of creating a modal dialog

• https://github.com/brainglobe/brainreg-segment/blob/34789347f3a908243dd991ff6120f7b3f232968f/brainreg_segment/layout/utils.py#L4
• https://github.com/brainglobe/brainglobe-napari/blob/double-click-row/src/brainglobe_napari/atlas_download_dialog.py#L11

We should merge these (and any others) and move them here.

As suggested by @adamltyson in https://github.com/brainglobe/brainglobe-napari/pull/49/files/1ee198c0232b29243fca8ce1bf302fdf2a2989d3#r1276300403