Allow sorting & reordering of pictures in galleries

[aurelienpierre]

When you create a gallery, pictures are sorted in the order in which you added them. That is entirely arbitrary since it matches no rational sorting (for example, by date taken, date modified or filename) and there is no way to manually change this arbitrary order once the gallery is created.

Since the sorting order is currently arbitrary, there should be a way to change it later, for example through drag and drop. In an ideal world, there would also be sorting options, and perhaps even filtering (by filename and extension).