Add properties editor

[zastrowm]

I, as a user, would like to be able to change the properties of controls so that I can customize controls to fit my needs.

We should be able to adjust things like:

• the text of the control
• the color of the control
• how the control is aligned (e.g. from the left, from the right)

Eventually these are things that would be nice to have as well:

• The ability to specify what happens when I click a button
• The ability to set the color/style of controls
• The ability to change what the controls are bound to
  • e.g. dynamic updates

Current Plan

• A property descriptor provides a list of properties for a specific type of control
• The property panel is simply a list of names to editable controls; all of the properties that are editable for the currently active control
• Each property provides the control that can edit it - this control is what's shown in the property panel
  • This allows the property panel to remain agnostic of how the properties are implemented
  • Eventually, we should expose more rich controls; for example, for text-alignment and other text related properties, instead of the grid of controls, we could expose more of the traditional buttons/rich-text controls like a button for bolding/italicizing/alignment. This is more of an eventual stretch goal than required for MVP
• Some controls need to have the concept of a "debounce"; we shouldn't always be updating all of the properties instantly - we need to take into account if the properties are being sent across the network.
  • this may be more of a concern for runtime-editing than design-time

Binding

Eventually we'll need a plan for binding. At the moment we could envision two types of binding:

1. Each property independently boundable
   • Ideal for when you have the concept of "variables" or "expression" for computing something
   • Also ideal for one-of bindings
2. A binding "context" where multiple properties are bound at runtime, depending on what's provided
   • Ideal for when something else can specify multiple properties (e.g. a spreadsheet cell) that can be adjusted at runtime. It would be a PITA to do (1) for each property that you want to bind, but perhaps if we provide a convenient way to start the properties bound, it would be acceptable.

We will probably implement (1) initially and potentially implement (2) as an app-specific thing. The important thing about (2) is that we expose a way to indicate the properties are overridden/sourced from elsewhere.