The InsertImage button shouldn't dictate where to insert images.

[jodator]

UI elements relays on Command to perform user actions like:

uiView.on( 'execute', () => {
	editor.execute( 'command' );
	editor.editing.view.focus();
} );

while the ImageButton also finds optimal insertion position.

IMHO the button should relay on this on ImageUploadCommand as it also depends on command's state.

I'd change this in #338.

// cc @pjasiun