Page layout editor: fix bad initialization on startup, which can crash pl_editor when trying to edit an item.
Minor other fixes.

Fix segfault when deleting last user path from eeschema libpaths and then pressing cancel.

remove global g_UserLibDirBuffer which was a project specific global, and no longer used with FP_LIB_TABLE support.

.kicad_mod files: always save the "last edited date" because only the footprint editor changes this parameter. Usefull to know if/when a footprint was actually modified.
Also fix a minor issue in footprint editor which always set the "modified" flag even when no change was made.