Added helper for document tree pre-order traversal.
Implemented data model for Tables.

Introducing dm.TableMatrix, a companion for dm.TableNode that provides a
non-sparse representation of tables with spanning cells.
Implemented CE Table.

ve.ce.TableNode provides some visual sugar via overlay to display the table
selection.
Providing Table manipulation via ve.ui.TableAction.
Icons and tools for table manipulations.
Add table example and include necessary source files in demo.
CE styles.
Custom sub-toolbar for table tools.
Provisional fix to prevent structural changes to a table via delete+merge.
Tweaking table toolbar.