CATROID-1394 Export projects to Downloads-directory

[Walcho1125]

Please enter a short description of your pull request and add a reference to the Jira ticket.
Export projects to Downloads-directory
https://jira.catrob.at/browse/CATROID-1394

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[Tamdav]

Looks good!

[wslany]

Please do not create a "Catrobat" subfolder in the "Download" folder. Instead, directly store the file in the "Download" folder at the latter's top level, without a subfolder. Thus, for ProjectOptionsTest.java also only delete the exported project, not the folder itself.

[Tamdav]

looks good

[wslany]

Thanks!