Add dialog for adding local stations

Merged Elias Projahn requested to merge johrpan/Shortwave:local-stations-ui into master

The new dialog encourages the user to add the station to RadioBrowser, but supports creating local stations as well.

This video shows the new dialog in action: shortwave-2021-05-09_00.36.28

Depends on !433 (merged).


Edited by Felix Häcker

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