Add abstract getAnnotation() method to AnnotationInspector

It was relied on, and all subclasses had one, but for some reason
it wasn't defined as an abstract method.

Change-Id: I6d48f8ee666bd339be87744840c6edb4abb56dbf
Always remove the annotation when the remove button is clicked

Remove weird check that prevented a removal from happening if the
subclass's getAnnotation() method returned null.

This caused a bug where if you inspected a link, typed an invalid
link target (e.g. '|'), then clicked the remove button (trash can icon),
the inspector would close but the link wouldn't be removed.
However, if you typed something that was a valid link target (or didn't
touch the input at all), the remove button would work as expected.

Change-Id: Ib6efc2a5827b109c6b38185e6d89b7bb29b13a75
Make autonumbered external links inspectable

When the target of an autonumbered link is changed to a URL, it's kept
as an autonumbered link and its target is updated. When the target is
changed to a MediaWiki page, the autonumbered link is removed and
replaced with an internal link with the text set to the target.
So for instance, if you inspect [http://www.example.com] and change
its target to "Foo", the result will be [[Foo]].

The core of this commit adds support for inspecting nodes to
ve.ui.LinkInspector. This support should probably move into a
class in between AnnotationInspector and LinkInspector, perhaps
called HybridInspector or something, but I'm deferring that for now.

LinkInspector allows changes to inspected nodes to be reflected either
as attribute changes on the node, or by replacing the node with something
else. MWLinkInspector uses this feature to replace the autonumbered
external link node with an internal link annotation when the target is
set to an external link.

Bug: 53505
Change-Id: Icb404af84c24574438e4de3ef05bbd1993b593f7
Merge changes Icb404af8,Ib6efc2a5,I6d48f8ee

* changes:
  Make autonumbered external links inspectable
  Always remove the annotation when the remove button is clicked
  Add abstract getAnnotation() method to AnnotationInspector