Accidental leading spaces in build directory input field causes build to fail

If you add a leading space to the build directory

 /tmp/untitled-qt5_dev_debug-Debug

You get the following warning:

The build directory needs to be at the same level as the source directory

This could be easily avoided by trimming leading and trailing spaces from the field.

It took me a while to spot what the problem was (I wasn't aware there was a leading space, as it was difficult to spot), and the warning (should probably be a build error rather than a warning) was not helpful.