main/templates: move language selection to the 'about' page

We don't want to have the selection occupy space in the main title.
main.carts: first draft of "cart" functionality

Inspired by the e-shop UI paradigm, created a "cart" data structure
inside the http session. This will be used to let some models "fill"
the carts of others.

We support multiple carts. Hence the user can work in multiple relations
simultaneously.

This is just a draft. Not much of the intended functionality yet.
main.carts: more framework for the carts

List the cart "actions" aside and below the list of items, let carts
be closed and opened with the correct "destination"
inventory: apply "cart" logic to inventories
assets: let the item actions only appear at sidebar, not inline
filter_form: context must be copied for filter form

The previous operator did nothing but reference the same context. This
meant that the parent context would be polluted with our values once
the filter_form had finished.

The problem became apparent when the filter form was moved to the top
of the layout.
generic_views/tmpls: remove trailing spaces
templates: move filter_form to the top of the "content" layout block
generic_views: cleanup the imports

Let pyflakes not complain
generic_views: the GenericBloatedListView, part 1

We want a powerful ListView, where all the goodies will go in.
company.urls: test the GenericBloatedListView

So far, worked as a drop-in replacement. Fine.
assets: use GenericBloatedListView for assets
generic_views: custom template for GenericBloatedListView, columns

the "columns" in the context will carry all information about each
column of the list view.
GenericBloatedListView: support for groupping, optimize

We now support a single-field (foreign one) groupping of the ListView's
results.
The API is as simple as a "group_by='fieldname'" argument to the view.
BloatedListView: configurable group columns
BloatedListView: icon before group rows
assets, locations: exploit the BloatListView
moves, company: move generic location filter to 'company', re-use
BloatedListView: allow extra_columns to come as init kwargs

Don't only expect them in "extra_context". This allows extra_context
to be defined at a different part than the columns.
assets.views: merge all Assets views in one class, reuse in Dept

We want one, fine-tuned asset view, which will be reused in assets
per Location/Department. Thus, move the definition to 'views.py',
make it a class and tweak it by inheritance.