Add failing test corresponding to issue 227.

Bug 227 is at last somehow reproducible, although not every
aspect of the bug is in the test.
Fix regression induced by previous `Decim` objects representation.

This made module `calculatrice` fail when dealing with decimals.
[Development framework] Minor enhancements to edit and search tools.
[Dev framework] Support nested lists and tuples in assertAlmostEqual.
[Issue FS#227] Fix bugs when a polygon creation fails.

When a polygon creation failed, edges and vertices were left in the
working sheet.
Edges and vertices creation process was altered, and some key modifications
were made in the sheet registration process.
Tests were added.

TODO:
* remove vertices and edges names from polygon styles (not needed anymore).
* vertices and edges registering must occure *after* polygon registering,
  and must be handled by the sheet (and *not* by the polygon itself).
  The reason is, a polygon might be used in the future as an intermediate
  construction object. It will then be linked to a sheet, but not registered
  (it won't have a name and won't be displayed). So, its edges
  (and vertices) shouldn't be registered either in the sheet.
* apply similar modifications to all automatically registered objects.
  (Vectors clones extremities for examples).
Refactor code concerning sheet registration process in geolib.

This mostly affects polygons.
Documentation was updated too.
Adapt `polyedre.py` to cope with registration process change.

Small refactoring of ._update() method too.