#15 Improved open dialog

[Tito Dal Canton]

The open dialog does not understand which files are Shapes and which are Sounds, so the user needs to make the correct choiche with the small popup menu to correctly open a file. This awkward behaviour should be fixed by doing a "pre-parse" of the files, to establish what they are, like it was possible with the NavigationServices in MacOS.

[Tito Dal Canton]

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This seems to require deriving some of the wx document-model classes, though I have yet to understand which ones.