iOS *.csproj <Import> is out of date

I switched to using the latest <Import> statements for the latest
Xamarin.iOS
Added some more path helpers.
We now use PathHelper.NormalizeDirectory() to get the correct seperators including a trailing one.
Cleanup any dot notation within the output and intermediate paths.
SongProcessor now creates the song output folder before conversion (NAudio will crash if it doesn't exist).
Using the MonoGame SongReader as refence, the duration should be written with type information.
WriteObject<T>() needs to write type identification for value types to match XNA behavior.
issue #1920: Song always loops on Android
PremultiplyAlpha should be set true by default, DefaultValueAttribute does not set the value.
Merge pull request #1924 from danzel/mgcb-alpha

PremultiplyAlpha should be set true by default
Pass the current Platform/Profile down when building content. This allows content writers to save things different for different platforms.
Merge pull request #1925 from danzel/mgcb-platform

Pass the current Platform/Profile down when building content.
Merge pull request #1921 from dmanning23/AndroidSongFix

issue #1920: Song always loops on Android
Merge pull request #1915 from jonathanpeppers/develop

iOS *.csproj <Import> is out of date
Merge pull request #1918 from SickheadGames/cbsong

MonoGame Content Builder Fixes
Merge branch 'develop' of git://github.com/mono/MonoGame into monogame-sdl2