QmlProject: Search for qmlviewer in configured Qt versions (if not found in $PATH)

If no 'qmlviewer' executable is found in the PATH, iterate through the
list of configured Qt versions and try to find a qmlviewer there. This
should help users configuring creator such to play with Qml.

Right now the first qmlviewer found in a Qt version is selected. A UI
to let the user select one explicitly was not possible any more (string
freeze).

This requires a dependency from QmlProjectManager to
Qt4ProjectManager.

Reviewed-by: dt

src/plugins/qmlprojectmanager/QmlProjectManager.pluginspec

@@ -18,5 +18,7 @@ Alternatively, this plugin may be used under the terms of the GNU Lesser General
         <dependency name="TextEditor" version="2.1.0"/>
         <dependency name="QmlJSEditor" version="2.1.0"/>
         <dependency name="Debugger" version="2.1.0" />
+        <dependency name="ProjectExplorer" version="2.1.0"/>
+        <dependency name="Qt4ProjectManager" version="2.1.0" />
     </dependencyList>
 </plugin>