Target .Netstandard 2.0 (#8)

* Fix build on non-windows platform

* Update to netstandard 2.0
Fix build on OSX

* Fix netstandard UWP API usage

* Update README.md

* Fix build

* Fix build

* Fix build on *Nix

* Remove Win stuff on nonwindows

* fix compilation

* Fixes

* Fix

Example/Example.csproj

@@ -1,16 +1,24 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
-  <PropertyGroup>
-    <OutputType>Exe</OutputType>
-    <TargetFrameworks>netcoreapp3.1;net5.0-windows10.0.17763.0</TargetFrameworks>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
+	<!--Note: Dotnet currently does not allow to build on non-windows platforms when a windows TFM is specified-->
+	<PropertyGroup Condition=" '$(OS)' != 'Windows_NT' ">
+		<TargetFrameworks>netstandard2.0;net5.0</TargetFrameworks>
+	</PropertyGroup>
+	<PropertyGroup Condition=" '$(OS)' == 'Windows_NT' ">
+		<TargetFrameworks>netstandard2.0;net5.0-windows10.0.17763.0</TargetFrameworks>
+	</PropertyGroup>
 
-  <ItemGroup>
-    <ProjectReference Include="..\DesktopNotifications.Apple\DesktopNotifications.Apple.csproj" />
-    <ProjectReference Include="..\DesktopNotifications.FreeDesktop\DesktopNotifications.FreeDesktop.csproj" />
-    <ProjectReference Include="..\DesktopNotifications.Windows\DesktopNotifications.Windows.csproj" />
-    <ProjectReference Include="..\DesktopNotifications\DesktopNotifications.csproj" />
-  </ItemGroup>
+	<PropertyGroup>
+		<OutputType>Exe</OutputType>
+		<Nullable>enable</Nullable>
+		<LangVersion>8.0</LangVersion>
+	</PropertyGroup>
 
-</Project>
+	<ItemGroup>
+		<ProjectReference Include="..\DesktopNotifications.Apple\DesktopNotifications.Apple.csproj" />
+		<ProjectReference Include="..\DesktopNotifications.FreeDesktop\DesktopNotifications.FreeDesktop.csproj" />
+		<ProjectReference Include="..\DesktopNotifications.Windows\DesktopNotifications.Windows.csproj" />
+		<ProjectReference Include="..\DesktopNotifications\DesktopNotifications.csproj" />
+	</ItemGroup>
+
+</Project>