Honor .nomedia file or similar

[newhinton]

Currently (afaik) all images are beeing shown. That leads to some images beeing shown twice when they are in a thumbnail folder. There could be a way to automatically hide folders/images that are in a folder beginning with a dot (.thumbnails) or if the folder contains a .nomedia file. This could be toggleable, like with 'show hidden files' in the files app.

A more elaborate way would be to manage exclusions via gui so that a user could choose a folder to be excluded, but imho honoring a nomedia file or the preceeding dot should suffice.

[skjnldsv]

This is already there #159