Allow deleting the whole feature from direct_select

[trygveaa]

When no vertices are selected, trash deletes the whole feature.

[adrianababakanian]

Thanks for opening this PR @trygveaa . The suggestion conflicts a bit with the logic outlined by the difference between the direct_select and simple_select modes -- which is not to say this can't be changed, just that we should consider the implications before moving forward.

Per the API reference for simple_select and direct_select, the former lets you "select, delete, and drag features" whereas the later lets you "select, delete, and drag vertices." This is reinforced by direct_select's fireActionableState method:

mapbox-gl-draw/src/modes/direct_select.js

Lines 22 to 28 in fb37f93

	DirectSelect.fireActionable = function(state) {
	this.setActionableState({
	combineFeatures: false,
	uncombineFeatures: false,
	trash: state.selectedCoordPaths.length > 0
	});
	};

The trash action is intended to only have effect when state.selectedCoordPaths.length > 0, which this PR would change.

Is the motivation here to remove the need to "click away" from a feature when it is in direct_select in order to then delete the feature with simple_select? I see the benefit this could provide to the user experience (i.e. fewer clicks), but also would like to consider the fact that this would blur the distinction between direct_select (more vertex-oriented) and simple_select (more feature-oriented).

That being said, both modes do allow features to be dragged.

[neodescis]

I think this change would be great. It's not always immediately obvious to the user whether they're in direct or simple select mode, especially after just creating a shape. If no vertices are selected, it's pretty strange to just do nothing when hitting trash/delete.