Ninja path validation doesn't check whether the found ninja is a file

Steps to reproduce:

1. Install Qt with ninja
2. Start AS with QtTAS
3. Go to the Qt settings menu
4. Set ninja path to "/Users/ollivuolteenaho/Qt/tools"

Expected:

    The path is not valid and cannot be set as it only contains a directory called "Ninja"

Actual:

    The path is accepted and the build will fail later because the "tools" directory doesn't contain a ninja executable