Logging, docs and style improvements.

No behavior changes.
Fix Reactive*ViewSource problems with ReloadData().

If a data source changes too many times, it is possible to break
a UI{Table,Collection}View.  Please see the docs of
lastCallToNumberOfSections introduced by this commit itself,
which fixes this problem.

This commit also adds hooks to Changing observables, thus releasing
bindings earlier than before.  I do not think that this is necessary
for correctness, but it does not seem to harm and may be beneficial in
avoiding needless work in some scenarios.
Less Timestamped mess.
Avoid DateTime in timestamps, use UInt64 instead.
Style fixups
Merge pull request #433 from reactiveui/reloaddata-sucks

Fix Reactive*ViewSource problems with ReloadData().
Add existing ReactiveList<T> properties to interfaces
Remove IsEmptyChanged - can be an extension method
Merge pull request #436 from onovotny/interfaces-pullup-members

Add existing ReactiveList<T> properties to interfaces
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'