Look at using portal for opening dropped files

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

The Flatpak version of Rymdport was having problems with dropped files and thus I made sure to bumped the permissions to the whole downloads folder. After reading a bit more into drag and drop for Flatpak applications, I think you might have to use the OpenURI portal to make use of the support for having dropped files automatically be passed into the sandbox.

Describe the solution you'd like to see.

If needed, implement the needed portal in rymdport/portal and use it here.