Add function getlibstatut and libstatut on task (fk_statut allready present on project_task table)

[ task #925 ] Add ODT document generation for Tasks in project module

upgrade were not yet received. So we prepare for GPL by uniformizing
licence text keys to GPL-3+. Will move later to AGPL if all answers are
positive.

Fix variable date property name, to be compliant with varaible naming
convention (create)
Add feature to shift task date when change project start date (option)
Add project start/end date on Project->Task View

Fix variable date property name, to be compliant with variable naming
convention (create)
Add feature to shift task date when change project start date (option)
Add project start/end date on Project->Task View