Make footprint ID into a generic library ID.

Rename FPID to LIB_ID as is now used as a generic library identifier and
is no longer specific to footprints.

Remove all mention of footprint from the new LIB_ID doxygen comments and
code.

Rename files fpid.h and fpid.cpp to lib_id.h and lib_id.cpp.

Rename fp_lib_table.keywords file to lib_table.keywords and adjust CMake
build dependencies accordingly.

Update all source files effected by the code and file name changes.

Update .gitignore for file name changes.

pcbnew/github/github_plugin.cpp

@@ -181,7 +181,7 @@ MODULE* GITHUB_PLUGIN::FootprintLoad( const wxString& aLibraryPath,
         if( local )
         {
             // It has worked, see <src>/scripts/test_kicad_plugin.py.  So this was not firing:
-            // wxASSERT( aFootprintName == FROM_UTF8( local->GetFPID().GetFootprintName().c_str() ) );
+            // wxASSERT( aFootprintName == FROM_UTF8( local->GetFPID().GetLibItemName().c_str() ) );
             // Moving it to higher API layer FP_LIB_TABLE::FootprintLoad().
 
             return local;