datetime: Avoid excessive copies in add_full()

datetime: Avoid excessive copies in add_full()

The current implementation of g_date_time_add_full() creates multiple
GDateTime temporary objects and unrefs them immediately; even with the
slice allocator this could result in a performance bottleneck,
especially if the atomic integer operations fall back to slow paths.

We can isolate the components of the add_full() operation and create
internal modifiers that operate on an existing GDateTime; this brings
down the number of GDateTime copies created from six to one.

While at it, the test suite for add_full() should have more checks for
roll-over of months and days.

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>