Specify custom folders for backup

Some apps store important data outside the `/Android/data/` folder. In such cases, AM should be able to back up the folders too. **Note that** there's no way AM knows in advance which data folders are used by an app. Therefore, the users have to specify them instead.

Due to the fact that each app uses different locations, this can't be added as a backup flag like others. Instead, the user has to specify the back up locations in the App Info tab. This could also be useful in future for other purposes.

Proposed workflow:
1. Go to **App Info** tab of an app
2. Click on the three-dots menu and choose **Specify other directories**
3. In the alert prompt, the user can insert a comma separated list of directories (which will also be displayed in the **paths & directories** section)
4. Now, select **back up/restore** either from the App Info tab or from the batch operations
5. Enable **custom data** flag
6. Click **backup** to make a backup of the folder(s)