Storage mounting

### ⚠️ This issue respects the following points: ⚠️

- [X] This is a **enhancement/feature request**. Not a [bug or question](https://github.com/gsantner/markor/issues/new/choose).
- [X] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it)_.
- [X] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [X] The wanted feature/enhancement is not present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Mordor**. Don't worry; Markor and Mordor appear as completely separate applications. You can install both side-by-side, and Markor's settings are not touched. If your desired feature is present, you don't need to open this issue. The change will be part of the next Markor update.)

### Description

It would be nice to have an ability to work with "virtual" filesystems (not sure about the correct name), that are created by other programs like EDS (app to open luks/encfs/veracrypt containers on android), and, if I understand correctly, are to be accessed via Android Storage Access Framework.

When these filesystems are created, they are being shown in default file manager in the left menu list below SD card and Internal storage. But in Markor they are neither visible in the list of places under top bar folder button, nor files from there can opened in Markor when being selected in android file manager, the app just crashes.

### Information

Android version: 9, Havoc OS 2.9
Device: markw (Xiaomi Redmi 4 Prime)
App Version: Markor 2.10.6 / Mordor 2.10.6-0942

### Source

git master branch

### Format / File type

Not specific

### Additional info / Log

_No response_