Fix some coverity warnings (not initialized members). Add comments. Better (more informative) title in dialog to download .pretty libs.

lib_dxf: Update to version 0.5.13 (previous: 0.5.11) due to a bug (memory leak) fixed in 0.5.13, which has also more comments, and try to fix most of coverity warnings (not initialized class members).
These members are now initialized, which also fix some other more serious coverity issues.

Pcbnew: very minor fixes and update:  update clipper version. uncrustify polytri/* and fix some warning compil.

Pcbnew:, libedit, Save lib as...: the new .pretty lib format is the default, instead of legacy .mod format. The legacy format is still selectable in the file selection dialog.

dxf import: minor enhancements in dialog. Update libdfxw from git, and uncrustify it. add a test dfx file