Fix cases of readonly on [Import]'s

[kornman00]

Both OpenDocumentResult and ShowToolResult imported IShell but their member variable was set to readonly, which resulted in a composition exception (member is not writable, see below). They are no longer readonly.

Additionally. no code was actually making use of the Show.Document/Tool helpers, which is likely how this went uncaught, despite the README.md providing an example of it for the HomeViewModel. ViewHome and ViewToolboxCommandHandler now use Show class instead of importing IShell themselves.

The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) Cannot set the value of 'Gemini.Framework.Results.OpenDocumentResult._shell' because the member is not writable. If the member is a property, it must have an accessible setter; otherwise, if it is a field, it must not be read-only.

Resulting in: Cannot activate part 'Gemini.Framework.Results.OpenDocumentResult'.
Element: Gemini.Framework.Results.OpenDocumentResult -->  Gemini.Framework.Results.OpenDocumentResult

[kornman00]

Additionally, changed mixed spacing to use spaces instead of tabs in the touched files, as that's what is in the .editorconfig

[kornman00]

Friendly bump @tgjones :)

[kornman00]

Please @tgjones :)?

[tgjones]

Sorry for the delay, merged now.