This commit is a split of the former graphview into a pivot view (the
tabular view) and graph view (the various charts).  The code was fully
rewritten.
* pivot view should be faster (better underlying data structure and
better algorithm)
* better integration with the web client (use of sidebar, of menu buttons,
bettor style)
* new feature: pivot view can be sorted
* new feature: pivot view cells can be clicked to open a list view with
the corresponding domain
* web_graph addon was removed, and the graph view code has been moved
to the addon web.
* the interaction with the searchbar was changed (no more column facet)
* better and simpler code for graph view
* mary corner cases in graph charts have been corrected (for ex, pie
chart with positive and negative numbers)
* graph/pivot correctly save their state when added to dashboard/saved
(state such as current measure, chart mode, column groupbys, ...)

In graph views, fields from the pivot table were not translated in the user language
We pass the context, containing the user language, to fields_get in order to get the translation
This is also done by the search view, in search.js line 1978 at this rev.

opw-616713

It was wrong.

XLWT only accepts sheet names smaller than 31 characters because that's the
excel limit.

Fixes #2548
Fixes #3318