qbs-config-ui should trim leading/trailing spaces away from entries

Right now it is possible to create an entry (key or value) with leading and/or trailing spaces, which in the latter case are impossible to detect visually without clicking and selecting the whole text. It's pretty easy to type a trailing space by accident and then have a seemingly correct entry cause all kinds of troubles.

The command-line qbs-config actually does care about spaces and trims them away.