Auto-open save dialog in the Deadlock install directory

[RedMser]

Auto-discover the install directory based on some heuristics.

I wrote some old code that did this once, it basically:

• Finds Steam
  • Default folder (C:\Program Files (x86)\Steam)
  • Registry key
• Checks built-in steamapps\common\Deadlock
• Queries the libraries vdf and checks each of them for steamapps\common\Deadlock

First try entering the addons folder if it exists, otherwise we could even prompt the user whether they want to create it (and update the gameinfo.gi automatically).

[RedMser]

Due to https://github.com/godotengine/godot/issues/99598 it is impossible to open the native FileDialog in another folder.

Committing the discovery logic autoload to master branch, but this can not be worked on until upstream fix.