Fix MenuItemSeparator

This pull request makes the MenuItemSeparator work, and some cleanups. It's almost working, just that:

- It has been misspelt as MenuItemSeperator.
- Even though it is in the source code, it is not being linked to `tint` binary because it is not in the gyp file.
- The file is using mixed spaces and tabs.
- One of the line has two semicolons.
- The getter is missing the return statement.