ISIS-778: removed EntityLinkSelect2PanelFactory

... because only ever used by ReferencePanel.  Instead instantiate panel directly.

(On way towards combining these two components).
ISIS-778: moved CSS from EntityLinkSelect2Panel to ReferencePanel
ISIS-778: reference panel simplification

... compact component uses EntityLinkSimplePanel rather than EntityLinkSelect2Panel (since simply need to render a link).
ISIS-778: moved html from EntityLinkSelect2Panel...

... into ReferencePanel.
ISIS-778: moving functionality into ReferencePanel

... from EntityLinkSelect2Panel.
ISIS-778: moving further functionality ...

from EntityLinkSelect2Panel into ReferencePanel

In addition, removing org.apache.isis.viewer.wicket.ui.components.actions.ActionInvokeHandler since does not seem to be used
ISIS-778: more stuff moved over

from EntityLinkSelect2Panel to ReferencePanel.  Specifically, the entityIconAndTitle link is now parented by ReferencePanel.
ISIS-778: tidy-up of ReferencePanel
ISIS-778: select2Field moved from EntityLinkSelect2Panel...

... to ReferencePanel
ISIS-778: further clean-up.

Although EntityLinkSelect2Panel has not quite been eliminated, reckon this is in a good-enough state to start on the @BulkParam functionality.
ISIS-778: removing ToDoItem#foo and #bar

... accidentally committed.