Add additional controls

[zastrowm]

I, as a user, would like to be able to add controls other than buttons, so that the user can provide other types of inputs like numeric values or textual values.

Possible list of controls:

• Textboxes
• Numeric up downs
• Labels
• Drop downs
• Checkboxes

Eventually it would be nice to support:

• Group boxes
• Images
• Radio buttons

[allenm84]

What about "list" type controls? ListBox, TreeView, GridView?

[zastrowm]

@allenm84 See #23 for list/treeview - there's not currently a design/plan, but we'll need them at some point

GridView I think would be taken care of by #24 in combination with #23 - I can see a use for #24 by itself, but it's also super-useful for #23 I think.