Allow adding to gallery from folder view

[aurelienpierre]

The only current way of adding pictures to galleries right now is from the "everything goes" app home page, which can be really messy since it's always sorted by date (in my case, I have several 100k pictures ranging over nearly 20 years — that means waiting for all pictures to load in view before accessing older ones).

It would be helpful to be allowed to select pictures to add to galleries from the folder view. Currently, this view does nothing more than the file app in a slightly prettier way (display thumbnails + show info when clicking the 3 dots icon).