In KIFACE_I::end_common save the 'common' configuration variables loaded in KIFACE_I::begin_common (the background color and the page border flag)

- Refactored the common part of cursor key movement and crosshair update
  in the various GeneralControl
- Add x10 movement with the keyboard (CTRL modifier)
- Avoid fixup of the cursor position by dummy mouse movements generated
  by cursor warping (original analysis and idea Chris Gibson)
- Do key handling in a way to permit sub-pixel cursor movement 

   to a new BOARD in the footprint editor.  This is PAGE_INFO::A4 and company.
   
*) Remove static storage of the BOARD in the module editor.

Change class PROJECT to use a generalized wxString in place of the
RETAINED_PATH items, so that new session and project specific strings
of any purpose can be saved there, for the life of a session.

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.