BREAKING: Add ContentPages and Invoke(sender)

[zadjii-msft]

BREAKING API CHANGES APPROACHING

Closes #307
Closes #238
Removes Command from ITag
Adds IDetailsCommand (to achieve the same goal as the above originally had)
Adds ITreeContent, based on a hunch
Adds ShowToast and Confirm to CommandResult too, but without UX yet

Extensions from before this change will need to be updated.

• The InvokableCommand change should be trivial - the helpers should abstract that delta away for you.
• The ContentPage change should be pretty easy to make.
  • Both MarkdownPage and FormPage are now just ContentPage
  • FormPage.Forms() -> ContentPage.GetContent()
  • MarkdownPage.Bodies() -> ContentPage.GetContent()
  • IForms become IFormContent. Methods become properties (not that bad)
• I'm only deprecating the old Markdown and Form pages - I'll fully remove them before we OSS, but I'll give everyone a couple weeks to port them.
• No one was using Tag.Command and it always seemed iffy at best - better not.