Streamline the project import workflow.

[starry-abyss]

The goal of this PR is to ease the steps required to import a project.

After clicking Import, the file selection dialog is immediately shown (previously one needed an extra button click).

The project file selection dialog now also supports selecting a directory, similar to Scan mode. Changes to Any mode of the selection dialog are because I wanted it to offer full functionality of both File and Directory selection modes.

Notes:

1. I had to still show the original dialog together in the background of file selection dialog, because otherwise the content of file selection dialog isn't rendered. The drawback is that when hitting Cancel in the file selection dialog, we see the original dialog, while ideally it shouldn't be shown on the first Cancel after clicking Import.

Maybe we should change the hierarchy of the dialogs to be siblings?

2. I was going to also merge Import and Scan, but not sure about the best approach to show Scan option in the UI of Import. Maybe it's for after this PR is merged.

3. The lines below are because in the original PR there was no issue described in 1). If we decide to ignore 1), then I think they can be removed:

// if not already shown
show_dialog();

Relaled: godotengine/godot-proposals#2483