[IMP] base: language export wizard

* move stuff around
* call write() from browse, correctly pass context to browse
* remove useless default to file name
* use contextlib with stringio
[FIX] base: export PO template files as pot

was already done when exporting to tgz, but not for po
[IMP] translations: parse views iteratively instead of recursively

also fix a pair of docstrings
[IMP] translations: simplify condition in qweb terms extraction
[FIX] translations: don't skip first line of translations when extracting module names

Used to be the first line was the CSV headers, the slice was left over after
these were removed from the source data. It probably didn't hurt (only issue
would be if the first module — alphabetically — has a single translatable
term), but it's just as clean not to have that.

Also removed now-unused variable (probably leftover of the CSV thing as well)
[FIX] base: incorrect translation mark
[FIX] web_diagram: can't mark empty strings for translations, gettext does not like empty msgids
[ADD] doc: basic translations guide
[IMP] various tentative improvements to translation code
[IMP] translate: improve management of targets in POT file
[IMP] doc: move translations to reference documentation
[IMP] translate: small, non-breaking code improvements
Merge pull request #3144 from odoo-dev/8.0-translations-export-xmo

Translation docs + code improvements and fixes