Allow pre-selecting file when opening file dialog

[crumblingstatue]

For example, if you already have foo.txt open, you could set it to be selected when opening the file dialog.
Useful when navigating a large directory with lots of files, to keep track of where you last opened a file.
pseudo-API: file_dialog.set_selected_path("foo.txt").

An alternative, less-flexible API would be to have an option to simply remember the last selected file.
pseudo-API: FileDialog::remember_last_selected(true).

[jannistpl]

Useful when navigating a large directory with lots of files, to keep track of where you last opened a file.

It is actually already implemented that the last opened directory is opened: https://docs.rs/egui-file-dialog/latest/egui_file_dialog/struct.FileDialog.html#method.opening_mode

Is that already enough for your use case?

[crumblingstatue]

This is about selecting a specific file inside the starting directory, not about selecting a starting directory.

[jannistpl]

Then we could also add a new variant LastPickedFile to the OpeningMode enum. We could then also save the file to the storage so that the last picked file continues to be selected after application restarts. Then, you don't have to worry about anything else on the application side.
What do you think?

[crumblingstatue]

If you think that's a good way to do it, then I'm fine with it.
I'm personally more of a fan of adding more general and flexible functionality first, like adding the ability to set any specified file as selected, but I'm not sure if you agree with that.

[jannistpl]

Then I would suggest renaming the method initial_directory to initial_path and allowing files there as well.

[crumblingstatue]

I like that idea!