Patterned document background option #3936

For maps and especially tilesets, it is often useful to be able to see transparent areas of the document. However, currently Tiled supports only solid colours as backgrounds - there is the background colour option, and the default map editor and panel background colour that's theme-dependent and shown on documents with no background colour set. These do not make it clear which pixels are transparent. While a lot can be done with them, I think that a zoom-independent chequered or otherwise patterned background (displayed in place of the current solid default) would make transparent areas read more clearly than any one solid colour could (at least without also blinding the user xP).

A comparison of such background patterns from a few image editors, with an image zoomed to 500% for reference: PyxelEdit: Diagonal lines, zoom-independent, fixed size, fixed colours

Photoshop: Checkerboard, zoom-independent, three size presets, customisable colours with several presets

Aseprite: Checkerboard, zoom-dependent (hurts clarity, but makes it useful as a size guide, something Tiled does not need), customisable size with several presets, customisable colours

Krita: Checkerboard, zoom-independent AND position-independent (cache-wrecker!), customisable size, customisable colours

I feel PyxelEdit's diagonal lines would be the best option, as they run little risk of resembling the content of tiles at any zoom level, whereas checker board patterns and squares are fairly common in tilemaps. However, it would be nice to be able to customise the size and colour of the lines - if not fully, then perhaps with small/medium/large size options (or perhaps a fixed size scaled by monitor DPI?), and light/midtone/dark colour options with the exact colours chosen based on the theme.