[oe] [RFC] change qmake2 to qmake4

[Valentin Longchamp]

As I have already explained in an email earlier today, some tools
(cmake for instance, with its FindQt4 Module) look for a qmake under
a certain name (that can be qmake or qmake4 ... but not qmake2).

This patch proposes to change this name. With this, all the other qt
tools consistently have the 4 at the end of their name.

Of course, all the recipes that inherit qmake2 should be changed, but
this is not done here since it's only a RFC to start a discussion

Best Regards

Valentin