Prompt user to open folder in "Current Window" or "New Window" (like IntelliJ)

Add an option to prompt the user whenever they open a folder, asking whether to open it in the current window or a new window.

**How it should work:**

Selecting Open Folder... shows a quick popup:
* ​This Window
​* New Window
​* Cancel

​**Why it's useful:**
This matches the workflow in IntelliJ / Android Studio. It makes switching between multiple projects much smoother and prevents accidentally overwriting the workspace you're currently working in.