Cleanup msv?VTKButtons???

[finetjul]

a) Minor design changes have been suggested as comments here: https://github.com/MSV-Project/MSVTK/commit/04ff834c39208d7fd8a705f0010ec73206189668

b) Apply MSVTK style to code:

• 2 space for indentation
• prefer SafeDownCast to reinterpret_cast/static_cast
• Use Superclass to mother class calls
• Prefer pre-increment over post-increment (++i instead of i++)
• #ifndef/#define follow the convention __filename_h
• Add space after if and for