decision: refactoring tooltips

The informations feeding the tooltip of an activity were collected and
combined by the DrawingArea. The tooltips system was not used the
wright way. From now on, the tooltips is used the proper way, and
methods have been added in order to delegate information query to the
dedicated classes representing the Model part of the MVC design
pattern.

Signed-off-by: Patrick Chabrier <patrick.chabrier@toulouse.inra.fr>
Revert "decision: refactoring tooltips"

This reverts commit 498d8ca7bda2e40eda55a520fd83b4143c3d1506.
Merge branch 'decision' of git://github.com/Chabrier/packages
all pkgs: update according the new Package API

- take into account the curr_package parameter for Modelling plugins
- update tests using a package instance rather than the singleton
forrester: modify the variable dialog

Variables item can be edited at the same level of expressivity than a
flow item. Variables can be linked with parameters item, compartment
item and exogeneous variables.
- The variable dialog has been removed.
- The flow dialog has been modified in order to be used for variable
and flow items.
- The variable item has been enhanced to carry more information.
(closes #19, #20)
muparser: fix missing doc/CMakeLists.txt

The file doc/CMakeLists.txt is missing and the CMake process fails. So,
we add an empty doc/CMakeLists.txt.