Rework bitmap system to load from archived PNGs

Bitmaps are now identified by an enum class instead of by pointers.
Bitmap loading and caching is now handled by a class in common, and
we no longer compile most bitmaps into the binary, so there is no
longer a bitmaps static library.

Instead, bitmaps are archived to a .tar.gz file which is installed
in ${KICAD_DATA}/resources/images.tar.gz

The source PNGs are checked in to Git as the original CPP files were,
so that people can build without the required dependencies to convert
SVGs to PNGs.

Initial support is also added for dark theme icons, although this
is not yet exposed in the GUI.

Stubs are present for multi-resolution image resources, but this is
not fully-baked yet and could use some refinement.

eeschema/dialogs/panel_setup_pinmap.cpp

@@ -72,7 +72,7 @@ void PANEL_SETUP_PINMAP::ResetPanel()
 void PANEL_SETUP_PINMAP::reBuildMatrixPanel()
 {
     // Try to know the size of bitmap button used in drc matrix
-    wxBitmapButton* dummy    = new wxBitmapButton( m_matrixPanel, wxID_ANY, KiBitmap( ercerr_xpm ) );
+    wxBitmapButton* dummy    = new wxBitmapButton( m_matrixPanel, wxID_ANY, KiBitmap( BITMAPS::ercerr ) );
     wxSize          bmapSize = dummy->GetSize();
     delete dummy;
 
@@ -132,7 +132,7 @@ void PANEL_SETUP_PINMAP::reBuildMatrixPanel()
             }
 
             int event_id = ID_MATRIX_0 + ii + ( jj * ELECTRICAL_PINTYPES_TOTAL );
-            BITMAP_DEF bitmap_butt = erc_green_xpm;
+            BITMAPS bitmap_butt = BITMAPS::erc_green;
 
             delete m_buttonList[ii][jj];
             wxBitmapButton* btn = new wxBitmapButton( m_matrixPanel, event_id,
@@ -156,23 +156,23 @@ void PANEL_SETUP_PINMAP::reBuildMatrixPanel()
 
 void PANEL_SETUP_PINMAP::setDRCMatrixButtonState( wxBitmapButton *aButton, PIN_ERROR aState )
 {
-    BITMAP_DEF bitmap_butt = nullptr;
+    BITMAPS bitmap_butt = BITMAPS::INVALID_BITMAP;
     wxString tooltip;
 
     switch( aState )
     {
     case PIN_ERROR::OK:
-        bitmap_butt = erc_green_xpm;
+        bitmap_butt = BITMAPS::erc_green;
         tooltip = _( "No error or warning" );
         break;
 
     case PIN_ERROR::WARNING:
-        bitmap_butt = ercwarn_xpm;
+        bitmap_butt = BITMAPS::ercwarn;
         tooltip = _( "Generate warning" );
         break;
 
     case PIN_ERROR::PP_ERROR:
-        bitmap_butt = ercerr_xpm;
+        bitmap_butt = BITMAPS::ercerr;
         tooltip = _( "Generate error" );
         break;
 
@@ -180,7 +180,7 @@ void PANEL_SETUP_PINMAP::setDRCMatrixButtonState( wxBitmapButton *aButton, PIN_E
         break;
     }
 
-    if( bitmap_butt )
+    if( bitmap_butt != BITMAPS::INVALID_BITMAP )
     {
         aButton->SetBitmap( KiBitmap( bitmap_butt ) );
         aButton->SetToolTip( tooltip );