Don't replace text in the find widget if there's text there

[ccordoba12]

During the Spyder 3.0 cycle someone added the functionality of automatically selecting the text under the cursor when you press Ctrl+F to launch a search.

That's very nice if there's no previous text in that widget, but a nightmare if there's already text some text in there. So we need to avoid this side effect while preserving the previous addition.