Make the Application fully composable

[fcollonval]

This follows the extraction of the plugin logic #703 to make the Application fully composable.

It also reduces the constrain on the shell to be a raw html element. The reason to remove the Widget constrains is to allow other frameworks to be used with the Lumino Application without using the widgets framework (as the context menu can be switched to a custom one).

[jtpio]

Just to have the full picture here, and to provide more context for the future when looking back at that PR.

The main purpose for allowing an HTMLElement directly would be for apps reusing the Lumino plugin system to not have to depend on @lumino/widgets to instantiate a placeholder shell widget, for example like in the following example?

lumino/packages/application/tests/src/index.spec.ts

Lines 21 to 24 in 0fb17ae

	const shell = new Widget();
	const app = new Application({
	shell
	});

[fcollonval]

Yes, although that the ContextMenu API is preventing to make @lumino/widgets an optional dependency;

lumino/packages/application/src/index.ts

Line 70 in 0fb17ae

readonly contextMenu: ContextMenu;