rose edit: fix capture middle click paste

This fixes a failure to capture a middle click paste event in the `gtk.Entry` widgets in rose edit.

@matthewrmshin, please review.

The change can be tested by opening an app that looks like this:
```
[env]
A=foo
```

selecting some text from a browser, terminal, or text editor, and middle click pasting it into the value widget for <samp>env=A</samp>. In the old code, no modification was noticed.