support Component DataSource

[tanbowensg]

What Changes?

The general idea is to reduce the DataSource's special logic and reuse the Component's mechanism.

Runtime level changes

• The DataSource now can be either a Trait+Component or a single Component.
• The metadata adds an isDataSource attribute to distinguish it from a generic Component or Trait. A Component or Trait with isDataSource will be shown in the DataSource list.
• dummy components also add an isDataSource.

Editor level changes

• Adding Trait such as state, fetch, etc. to Component is no longer prohibited.
• All DataSource reuse Component's forms. For special forms such as Fetch Trait, they should be wrapped into Widgets.
• DataSourceList reuse StructureTree components and have the same capabilities as component trees.
• When creating a new DataSource, two methods are provided, either directly creating a Component that has isDataSource or selecting a Trait to create with dummy Component.
• It is best practice to have only one Trait for a DataSource, but adding more than one at the UI level is not prohibited.

#639 #571