Get the geojson example working again

fdbbfa4a6723b816478ee011ab4098e5332dc2f5 removed MapObjectView, but this
example continued to depend on it. This reverts parts of commit
be7cbed7411d024d178377bd327d5916c80e02a0. Also:
- use QtCore.StandardPaths rather than Qt.labs.platform
- remove the widgets dependency
- use FileDialog.nameFilters to read and write geojson files

Let's assume that FileDialog.selectedFile will actually output a QUrl
to save the file to, as opposed to a plain path, since the dumpGeoJSON()
function is calling toLocalFile() on it. At least that works with
native dialogs.

Task-number: QTBUG-108455
Change-Id: I1f14c6425115aa95ac2080438ff0c3a24b1b7196
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

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