Others: fix very minor issues.

page layout: if the environment variable KICAD_WKSFILE points a S expr page layout descr, this descr is used instead of internal descr. Mainly for testing purposes.
Eeschema, dialog edit component: minor enhancement (the last selected notebook page is remembered during a session)

Eeschema, dialog netlist: fix a bug when removing a plugin panel, when is is not the last.
dialog netlist:Add a predefined command string for python scripts, when creating a new netlist plugin entry.

All: use a S expression to describe the page layout (title block and grid references). Work in progress.
This should  allow users to define their own page layout.

* Add footprint library table loading to footprint editor.
* Overload FOOTPRINT_INFO::ReadFootprintFiles() to read footprints from the
  footprint library tables.
* Fix a bug in FP_LIB_TABLE::IsEmpty() when the table has a fallback table.
* Add code to FOOTPRINT_EDIT_FRAME to use footprint library tables.
* Add an optional build time version of PCB_EDIT_FRAME::loadFootprints() to
  populate netlist footprints from footprint library table.
* Remove adding footprints to board whenever GetModuleLibrary() is called and
  move loading locally as required.
* Add missing source file license comments and coding policy fixes.

Pcbnew: fix bug in drag module: connected tracks were not dragged.
worksheet: code cleaning.