The EmptyReason property should support polymorphic types

[Kittyfisto]

At the moment, Tailviewer defines an enum with a few reasons as to why a log file is empty. However as soon as plugins start adding support for custom log files, there can be many more reasons why a log file is empty.

Plugins should be able to define their own reasons as to why a log file is empty and Tailviewer should just roll with it and display said reason.

Required changes:

• [x] Replace ErrorFlags with an IEmptyReason interface:

  interface IEmptyReason
  {
  string Statement {get;}
  string Explanation {get;}
  IReadOnlyList<string> Options {get;}
  Geometry Icon {get;}
  }

• [x] Log Entry Viewer displays these values as is
• [x] Change the GeneralProperties.EmptyReason property to use the new interface, a null value is to be interpreted as the source not being empty
• [x] Create a static class "EmptyReasons" which (similar to GeneralColumns and GeneralProperties) collects the reasons that Tailviewer cares about why a log file is empty: SourceDoesNotExist and SourceCannotBeAccessed