Linux: file dialogs

[romgrk]

This PR implements linux file dialogs and open/reveal actions.

Open folder	Reveal path

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[Eiji7]

Sorry if it's wrong place to ask, but I just found your PR and would like to know if the design is already decided. If not I would like to ask about support for ~/.local/share/user-places.xbel and similar files. KDE Plasma in this example have a XML file from which we can fetch an information about:

1. User directories (home, documents, downloads etc.)
2. Block devices like USB drives or partitions
3. Network places
4. Recently used

Having support for all user places regardless of used Desktop Environment would be definitely a great addition.

Here is an example screenshot from a Dolphin app.

[romgrk]

The xdg-desktop-portal backend would handle those concerns.

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[mikayla-maki]

Thank you so much!

[Sergekarr]

@cla-bot check

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