Create the mainwindow when setting the qml path, if needed

This fixes a regression introduced by bc5bd724372050c1dcf08b9c50ec01ce711cc484 for Nemo's lipstick-colorful-home.

mainWindowInstance() was never called anymore, so _mainWindowInstance was never created. This could also be fixed by calling that from the lipstick-colorful-home's main.cpp, but I think from an API pov having it created by lipstick is better, also because of the naming of the function, which makes it look like a simple getter.