feat(editor): refactor import and save

This commit refactors the model import. It provides the following
improvements:

- model files are not loaded twice during import
- import is done when Bpmn2Editor#setInput() is called, only
- save as is reactivated
- editor automatically closes if opened file can no more be found

- various fixes for exceptions during handling of non-workspace files
feat(editor): add automatic refresh / multi page

This commit adds the ability to automatically refresh diagrams on
changes and employs it in a multi page editor.

Details:

* refactor the import / refresh logic: now contained in a diagram
refresh
  behavior and executed once the diagram editor activates only

* rewrite import so that it automatically clears the scroll util in
  case it is present

* fake resource pattern employed to map file system resources to the
  workspace: for each fs diagram, a workspace resource is created,
linked
  to it. this allows us to employ validation, exists checks and attach
  change listeners to a resource

* clean up old code

* add a multi page editor that shows design and source view. enable it
  in plugin.xml
fix(outline): make refresh not deselect elements

This commit disables propagation of selection events temporarily during
refresh. This fixes issues with loosing focus on selected diagram
elements while the outline is open.
feat(editor/multi-page): add out of workspace sync

This adds functionality to detect out of workspace changes to BPMN 2.0
model files by comparing file and resource timestamps.

This is needed because the workspace synchronization is broken for
linked resources.
feat(editor/multi-page): add conflict warning on page change

Upon page change, a message asks to save the previously selected editor
page before continuing to work on a new one. This is needed to prevent
editing conflicts as the editors (Source, Design) are not synchronized
with each other.
fix(editor): correctly check for editor input equality