First implementation of route steps plugin.

1) Created plugin in Osmand/Plugins/Osmand-RouteSteps.
2) Registered plugin in OsmandPlugin.java - InitPlugin()
3) Added dialog to view all route dots in ContextMenu.
4) Save status of user selection.
5) Save changes to gpx file.


Known issues:
GPXUtilities.writeGpxFile() somehow doesn't save extentions so user selection are not being saved to gpx file.
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