Disable event manager when editing is disabled

[bantic]

Previously, manual (as opposed to programmatic) editing was being
prevented for an editor after calling #disableEditing by setting
"contenteditable" back to false for the editor's element. This prevents the
browser from sending most of the edit-related events (keyup, keydown,
etc) that occur when editing manually. But some events (like paste) are
still emitted by the browser even though "contenteditable" is false.

This change calls EventManager#stop when editing is disabled, and
internally the event manager ignores all events when it is stopped.

Also some rearrangement

Other changes

• Change editor's internal isEditable prop to default to true rather
  than null (incidentally allows a user to specify that an editor
  should start disabled by passing in that prop to the constructor)
• Add test for triggering paste event in a disabled editor
• add assertion assert.isBlank
• better assertion actual/expected values for hasElement and hasNoElement
• add test helper getData (for parity with the setData in
  element-utils and to avoid using jQuery's .data() method)
• consolidate the editing-disabled tests into a new suite
• make editor#render call disableEditing or enableEditing after
  it sets hasRendered to ensure that the event manager is stopped if
  the user calls editor.disableEditing() before calling
  editor.render()

Fixes #572

[bantic]

released in v0.10.18