This introduces a much more consistent set of events within the editor
for whenever a element is added, removed or modified.
The events emited on the `RED.events` event system. The event names
take the form: `"<thing>:<action>"`.
`<thing>` can be one of:
- nodes
- flows
- subflows
- groups
- links
`<action>` can be one of:
- add
- remove
- change
The payload of the events is the object in question.
There is also:
- flows:reorder - when tabs are reordered. Payload is array of flow ids.
- workspace:clear - when the workspace is emptied - part of switching projects
The `nodes:change` event was already used by RED.nodes.dirty() to cause
the Deploy button to become active. This renames that event to:
- workspace:dirty - Payload is boolean flag for the dirty state
This commit also updates the Palette to use the subflows:change event to
only redraw subflows that have actually changed rather than refresh them
all whenever one of them *might* have changed. This removes a noticable
flicker of the icon which was needlessly being redrawn.
