Added the most fundamental and basic 'Audio'.
Added the 'Presets' section (still under construction)

Now the audio source (PureToneSource.cs) plays the audio
using random pauses instead of the continuous tone,
so the test is more in line with medical recommendations.
Added the 'Add item' screen/dialog on Presets screen (with validation)

Added the 'tap' handling on SavedFilesViewModel.cs, now it fires an event
that is handled by the main pivot view - activates the 'current' view model and switches
the pivot page.

Changed the Frequency type from double to int