[IMP] hr_timesheet, project: various usability-related improvements

hr_timesheet: merged use_timesheets and invoice_on_timesheets fields on account.analytic.account
that basically tells the same thing (project using timesheets). Propagated this change in
the various views using it.
project: various improvements in views to ease the user experience.
Merge branch 'master' of https://github.com/odoo/odoo into master-linux64