Added helper for document pre-order traversal.
Data model implementation for tables.

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

ve.ce.TableNode provides some visual sugar via overlay to display the table
selection.
Actions for manipulating tables.

ve.ui.TableAction contains actions as well as a generic table manipulation API.
The API does only depend on DM and could be moved to DM world someday.
Icons and tools for table manipulations.
I18n for table tools.
Add table example and include necessary source files in demo.
Custom sub-toolbar for table tools.
Provisional fix to prevent structural changes to a table via delete+merge.