use a different (fake) enum namespace for shiboken & shiboken2

[devernay]

See discussion in #698:

• Support for building with Shiboken2 in Engine #697 and Shiboken2 postmerge fixes in Engine #704 introduced changes that broke shiboken1.
• we want to keep sbk1 compatibility as long as possible, because sbk2 means switching to Qt5, and this won't happen before 2.6 or 3.0
• this PR fixes that, so that both shiboken1 and shiboken2 can be used.
• it puts all enums in the fake NATRON_ENUM namespace, so that the shiboken/shiboken2-generated code is OK, and when compiling Natron this is defined to NATRON_NAMESPACE (see Global/Macros.h)

Things to watch for:

• a lot of the pypanel_wrapper code is not generated anymore, why? same for a few other Gui classes

If this is merged, next operations are:

• rework Support for building with Shiboken2 in Gui #698 the same way
• move all shiboken-generated source files to Engine/Qt4/NatronEngine end Gui/Qt4/NatronGui
• update instructions in INSTALL_LINUX and INSTALL_MACOS to regenerate these
• generate shiboken1 and shiboken2 files, and check in the generated sources

[YakoYakoYokuYoku]

Could generate the sources with both my Qt5 environment as well as a Qt4 devel VM box, you can go ahead.