Disable setting Content for ViewHosts in Design Mode

This allows you to set Content in design mode to a UserControl but allow it to
be overwritten at runtime, so you get a design experience
Fix ancient Contributing File
Version bump to 5.4.0
Add in missing NUnit shim for XUnit
WinForms command binding should affect Enabled
Merge remote-tracking branch 'origin/pr/443'
Update NuSpecs to pull in Ms RX
WinForms command binding return higher affinity than XAML

There exist System.ComponentModel.Component subclasses which would be useful for binding as a command e.g. MenuItem, NotifyIcon, ToolBarButton, however others are not e.g. Timer, Process, SerialPort.

This change ensures that falling back onto hasEventTarget or defaultEventsToBind for the useful ones won't result in ReactiveUI.Platforms/Xaml/CreatesCommandBinding.cs (which returns equal affinities for these cases) attempting to handle binding instead.
Merge pull request #447 from rzhw/patch-2

WinForms command binding return higher affinity than XAML
Fix typo
Merge pull request #444 from onovotny/rxui6-nuget-fixes

Update NuSpecs to pull in Ms RX
Fix Rx references and create Net45 sln configs - enables tests to build/run in VS
Enable BindCommand to work with nested VM's -- x.ViewModel.NestedViewModel.Command.
Include Events_Net45 in the config
Add a few more unit tests around commands
Merge remote-tracking branch 'origin/pr/450'
Merge remote-tracking branch 'origin/master' into rxui6-master
Merge remote-tracking branch 'origin/rxui6-master' into use-splat