fix(react-core): prevent Getter losing on Plugin adding

[DmitryBogomolov]

What happens

Plugin, Getter and Template have internal objects (this.plugin) which ordered within PluginHost internal object (this.plugins) based on corresponding positions.
Position originates from component index within its parent children.

For this tree

<PluginHost>
  <PluginA />
  <PluginC />
</PluginHost>

the list is the following

[ A(0), C(1) ]

When tree changes to

<PluginHost>
  <PluginA />
  <PluginB />
  <PluginC />
</PluginHost>

PluginB is created (internal object is created and added to list) and PluginC is updated (index changes).
During PluginB creation its internal object is created and has position (1). But PluginC is not updated by that moment and its internal object position is (1) too.
Such position collision is resolved in such a way that B(1) replaces C(1). The list is

[ A(0), B(1) ]

---

Here is a sample of React behavior.
Click the Change button and note that C2 and C3 are created C4 and C5 have outdated indexes.

What is done

• key property is used to force recreation all items going after newly added
• unregisterPlugin fixed so that it does not accidentally remove last list item

Because of key PluginC is destroyed (and C(1) is remove from list) and created (and C(2) is added to to list) after PluginB is created. So the list is

[ A(0), B(1), C(2) ]

Since registerPlugin allows the case when other list entry is removed (when there is position collision), unregisterPlugin should allow the case when no entry is removed (because it could already be removed in registerPlugin).

[DmitryBogomolov]

Fixes #1971.