Used cached footprints without check in preview

Between the time the user loads the footprint preview in cvpcb or Add
Footprint in pcbnew, we don't need to verify cache integrity to display
the footprint.  We use the cache copy when displaying until we close the
selection window and re-open.  This limits our need to iterate over the
full directory on every footprint display.

Alternate solutions for v7 should include looking into libfswatch for
change detection

cvpcb/display_footprints_frame.cpp

@@ -405,7 +405,10 @@ FOOTPRINT* DISPLAY_FOOTPRINTS_FRAME::GetFootprint( const wxString& aFootprintNam
 
     try
     {
-        footprint = fpTable->FootprintLoad( libNickname, fpName );
+        const FOOTPRINT* fp = fpTable->GetEnumeratedFootprint( libNickname, fpName );
+
+        if( fp )
+            footprint = static_cast<FOOTPRINT*>( fp->Duplicate() );
     }
     catch( const IO_ERROR& ioe )
     {

pcbnew/footprint_preview_panel.cpp

@@ -138,7 +138,12 @@ bool FOOTPRINT_PREVIEW_PANEL::DisplayFootprint( const LIB_ID& aFPID )
 
     try
     {
-        m_currentFootprint.reset( fptbl->FootprintLoadWithOptionalNickname( aFPID ) );
+        const FOOTPRINT* fp = fptbl->GetEnumeratedFootprint( aFPID.GetLibNickname(), aFPID.GetLibItemName() );
+
+        if( fp )
+            m_currentFootprint.reset( static_cast<FOOTPRINT*>( fp->Duplicate() ) );
+        else
+            m_currentFootprint.reset();
     }
     catch( ... )
     {

pcbnew/generate_footprint_info.cpp

@@ -49,7 +49,8 @@ class FOOTPRINT_INFO_GENERATOR
     wxString      m_html;
     FP_LIB_TABLE* m_fp_lib_table;
     LIB_ID const  m_lib_id;
-    FOOTPRINT*    m_footprint;
+
+    const FOOTPRINT*    m_footprint;
 
 public:
     FOOTPRINT_INFO_GENERATOR( FP_LIB_TABLE* aFpLibTable, LIB_ID const& aLibId )
@@ -71,8 +72,8 @@ public:
 
         try
         {
-            m_footprint = m_fp_lib_table->FootprintLoad( m_lib_id.GetLibNickname(),
-                                                         m_lib_id.GetLibItemName() );
+            m_footprint = m_fp_lib_table->GetEnumeratedFootprint( m_lib_id.GetLibNickname(),
+                                                                  m_lib_id.GetLibItemName() );
         }
         catch( const IO_ERROR& ioe )
         {

pcbnew/plugins/kicad/kicad_plugin.cpp

@@ -2284,7 +2284,7 @@ FOOTPRINT* PCB_IO::FootprintLoad( const wxString& aLibraryPath, const wxString&
 
     if( footprint )
     {
-        FOOTPRINT* copy = (FOOTPRINT*) footprint->Duplicate();
+        FOOTPRINT* copy = static_cast<FOOTPRINT*>( footprint->Duplicate() );
         copy->SetParent( nullptr );
         return copy;
     }