QmlProject files: Make "paths" property an array

Instead of letting the list of files being a (comma separated) string
it's now a full JavaScript array. E.g.

  paths: ["file1", "file2"]

This unifies it with e.g. the libraryPaths property

tests/auto/qml/qmlprojectmanager/fileformat/tst_fileformat.cpp

@@ -153,8 +153,8 @@ void TestProject::testFileFilter()
             "import QmlProject 1.0\n"
             "Project {\n"
             "  QmlFiles {\n"
-            "    paths: \"file1.qml,\n"
-            "file2.qml\"\n"
+            "    paths: [ \"file1.qml\",\n"
+            "\"file2.qml\" ]\n"
             "  }\n"
             "}\n");