[IMP] hr_holidays : split hr.holidays into hr.leave and hr.leave.allocation

Purpose
=======

Have a clear distinction between leave and allocation requests instead of
using the same model to mix 2 different concepts.

Splitting the model will allow different business code to be run on each
model as allocations and leaves are not exactly the same thing; this
will add some code but simplify future improvements

Specification
=============

1/ Completely separate the models

Model hr.holidays has been split into leave.request and
leave.allocation.

2/ Make reporting working again by using an SQL view aggregating data
from leaves and allocations

A new report has been added in order to aggregate
datas from both allocations and requests. The views have been modified
accordingly.