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### Problem description
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implemented, or link to an existing issue describing it -->
see #723
### Implementation description
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`xdg-desktop-portal-wlr` is not relevant because it does not provide the
FileChooser interface.
`xdg-desktop-portal` needs the `WAYLAND_DISPLAY` env var on wayland and
the `DISPLAY` and `XAUTHORITY` env vars on xorg. If the dbus user
session bus is not run in a way that it already gets those variables,
they must be given via `dbus-update-activation-environment(1)`.
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### Additional things
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fixes#723
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### Problem description
- Fixed disk provider not working for linux
### Implementation description
- Used ioctl instead of fstat
- Fixed buffer issues
---------
Co-authored-by: WerWolv <werwolv98@gmail.com>
### Problem description
Currently, the providers use the method `isSavable()` to determine both
if they can use "Save" or "Save as".
This behaviour is problematic because some providers may need to be
saveable but not saveable as: for example the view provider. The
original provider may not allow to be saved.
### Implementation description
I separate these two behaviour by creating another function:
`isDumpable()`, that return true by default but can be overridden by the
provider to return false, if the provider should not be dumped in any
way.
### Additional things
While I was at it, I also marked "export" operations as needing the
"dumpable" flag. That way, we can't accidentally export the whole
address space of a process as base64.
I also added documentation for these some functions in Provider
