set qml tool as the system viewer for qml files

The qml tool added by this change https://codereview.qt-project.org/#change,43540 is intended to be used as a system qml viewer application. It can be launched via the shebang mechanism in Unix-style shells, for example (a qml file can start with #!/path/to/qml), and if the user sets up QML as the tool for qml files to "Open with..." then it becomes possible to double-click QML files in any file manager/finder and view them with the qml tool. So the next step would be for the installers to set up this association by default. On platforms where it's possible, there could be two associations: the qml tool to view the file and Creator to edit it.