[Feature] Mobile: improvement of selection album to backup

[Solarunit]

The feature

Problem.

At the moment mobile app searching for the all folders with images:

• it populates a lot (hundreds) of folders you don't need e.g. all folders with music albums containing covers
• app becomes slow to scroll because list is too long
• it hard to select the folders that actually needed to backup
• it generates a very long list of excluded folders, information that not needed

Suggestion:

It would be nice to select only particular folders that need to backup manually maybe from internal file manager; don't auto-scan all system for images, similar to Simple Gallery.

Platform

• Server
• Web
• Mobile

[bo0tzz]

Addition from #6185:

I think nearly nobody wants to upload all assets from all folders => do not list the "Recent" (Android)
pre select the camera as default, most users want to save it and only it
maybe: list selected folders on top of not selected folders, they could be hard to find, if there are a lot of potential folders
maybe: show a hierarchical presentation of folders, because some apps create a whole hierarchy of folders containing possible assets, we don't want to backup.
if folders are showing in a flat list, like now, which is also fine, then there should be an easy way (easy for non nerds, but for normal users, not knowing special Unix command syntax), to exclude folders and their subfolders. A kind of file browser should be used to select folders to be excluded together with their subfolders.

[tomiko23lol]

• Please implement tree folder structure, listing all available folders is really bad idea. Also If I select parent folder I would expect that every child will be included too. Ideally leverage internal file manager for that.
• Problem > when I close app and open it again, I will click on cloud button and folder selection and upload progress is not there. My settings are not being saved or what?

[jaecktec]

I did a small check and it can be easily implemented by using the existing file_manager dependency. No new permissions or dependencies are needed.

(I have never worked with flutter, but I could hack together a small dialog - a more experienced dev needs to build a PR that doesn't look like garbage)