Fixes #355: Add component duplication support

[vldmrkl]

This PR contains a fix for #355.

It adds a new item "Duplicate as..." to the context menu. If the user selects this option, the app will prompt the user to type a name for this component and select location for it. Once the user saves the file, it's going to be opened in the Component Editor.

Implementation:

1. Check the extension of the selected file.
2. Add menu item "Duplicate As..."
3. onClick it shows save dialog.
4. Try to copy the item to the new path.
5. If successful, open a newly created file. Otherwise, it will show an alert with message "Couldn't copy component to..."

[dabbott]

Nice! I tried it out and it works well. I had a bunch of feedback on small things, but overall looks good.