Pcbnew:  enhancements for blind/buried vias placement ( separate keys to add a buried via or a through via), and save" blind/buried vias allowed" option in .kicad_pcb file
Minor code cleaning.
Allows compiling under wxMSW-2.8.12, only for test purposes, because there are serious bugs for Kicad under wxWidgets version < 2.9.3.  (compil warning generated for these versions)