[FIX] tools.translate: when loading entries from a PO file, use also the POT.

On Launchpad, as commented on bug 933496, PO entries (and their comments) are
shared between series. This mean that e.g. the 7.0 series can have the wrong
`reference` comments (those beginning with #:) as they would be copied from say
the trunk series. Those `reference` comments are used to import translations
and look them up.

This patch adds a few lines of code to tools.translate so that targets
defined in the POT `reference` comments are used in addition to those from the
PO file.

Also adds a test module to validate the new behavior.

This patch stems from:
 - the 6.1 branch by Vo Minh Thu:
    https://code.launchpad.net/+branch/~openerp-dev/openobject-server/6.1-fix-po-targets-933496-vmt
 - the 7.0 port by Numerigraphe:
    https://code.launchpad.net/~numerigraphe-team/openobject-server/7.0-fix-po-targets-933496-vmt
[FIX] tools.translate: addons path resolution failed with duplicates

When the addons_path config contained the
default path, the system was working with
an empty addons_path in the middle of the
other paths. This empty one matched for all
files, making all files appear to belong to
the root path component, e.g. `home` if the
root path is in /home.
Refactored a bit to avoid duplicates and
redundant path calculations.
[I18N] Update all 7.0 translation templates with latest terms and annotations

Total new terms: 168
Total deleted terms: 95
Total identical terms: 16329
(Some modules skipped, typically all l10n_* modules)
[I18N] Update 7.0 translations with latest changes on Launchpad

See also https://github.com/odoo/odoo/wiki/GitHub-Transition#translations
[MERGE] Forward-port of 7.0 up to 85d2b45