[Feature request] Support multiple folders in the same backup

[ngosang]

First of all, thank you for this amazing App. Its the best app I have found for Restic / Android. You should publish it in the marketplaces, it will be more visible.

I have a smartphone with Android 13. The app is working well but the configuration is a bit more complicate than it should be.

Use case: I want to backup all the files in my phone. I can't not select the main folder of my phone (where DCIM and Android folders are) because privacy or permissions. I don't care about permissions but Android don't let me.
To solve this I'm creating different backups. One for DCIM, one for Android folder... It works but instead of having 1 backup, I have a lot of different backups and if I want to restore my phone I have to restore many backups one by one.

My proposals:

1. Find a way to select the main folder. I don't see the reason it's not allowed. This will be perfect combined with Ignore patterns #86
2. Allow the user to select more than one folder in each backup. Convert the folder path into a list with multiple paths. All files from all paths will be included in the same backup. It's supported by Restic using restic ... backup folder1 folder2 ... Another way of doing the same is => https://restic.readthedocs.io/en/stable/040_backup.html#including-files

[ngosang]

Workaround to select the "main folder" until this feature is implemented.

1. Select any folder
2. Edit the text in the "path" textbox and set /storage/emulated/0
3. Run the backup

If you have any problem with permissions make sure the app has permission to MANAGE_ALL_FILES