Add default keyboard shortcuts to various actions in the FileSystem dock

[Calinou]

• Follow-up to https://github.com/godotengine/godot/pull/90421.

Some of these shortcuts are inspired by Visual Studio Code's defaults:

The defaults are:

• Ctrl + Alt + C: Copy Absolute Path (like in VS Code)
• Ctrl + Alt + Shift + C: Copy UID
• Ctrl + Alt + R: Open in File Manager (like in VS Code)
• Ctrl + Alt + E: Open in External Program
• Ctrl + Alt + T: Open in Terminal
  • Note: On Linux distributions that have Ctrl + Alt + T as a default shortcut to open a terminal, the system shortcut takes priority over this one.

The "Open Containing Folder in Terminal" text was shortened so that the context menu doesn't become too wide with the default shortcut annotations.