Add mode as standardProperty to configurator
Refactor standardProperties
Add default mode for all configs in configurator

The global default mode is 'editor'.
Change heads only when requested in state

The selectToken function takes an additional param to announce the
possibility of a head change now.
Change head in token directive in editor mode only

The knowledge of whether we can change heads or not is currently
derived from injecting the depTree plugin explicitly.
Check the comment on the status of this solution.
Add ability to customize globalDefaults
Merge pull request #198 from latin-language-toolkit/view_mode

Adding the concept of modes