doxygen.cmake: create a doc target for every plugin
doxygen.cmake: create a doc target for every plugin

This commit changes the cmake script to generate a documentation target
for every plugin in plugins/, instead of only the enabled ones.

Also, minor adjustments.


Former-commit-id: 09a447b80c1bc0da145c4e262c39d1dff4defa6b
hdeling/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa