Support opening file in Rymdport to start send

[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?

It can be very useful to be able to right-click on a file, folder or multiple files in the file dialog and then open them in a specific program. This way we can let a super start a send very easily.

Describe the solution you'd like to see.

We need to implement the send portion of #75 and somehow implement the file-manager integration for all platforms.

Linux and BSD should be quite straight forward. I can probably get that wired up quite quickly for both the Flathub package and release binaries (the latter will require building on the work in fyne-io/fyne#3958 upstream).

Getting it to work on macOS and Windows might have to wait as I have no idea how to do it and it will require coordination with Fyne on how it can be made in a simple manner.

Systems to implement this for:

• Linux and BSD
• macOS
• Windows

[Jacalz]

It looks like this could be as simple as just adding %u at the end of the Exec=rymdport field in the desktop file.

[Jacalz]

It looks like we may want to use %U (if we add support for uri:s) or %F in our case: https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s07.html

[Jacalz]

Linux and BSD support will be included with v3.5.0. Windows and macOS support will have to come later.