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.

pcbnew/board_stackup_manager/panel_board_stackup.cpp

@@ -103,7 +103,7 @@ PANEL_SETUP_BOARD_STACKUP::PANEL_SETUP_BOARD_STACKUP( PAGED_DIALOG* aParent, PCB
 
     // The grid column containing the lock checkbox is kept to a minimal
     // size. So we use a wxStaticBitmap: set the bitmap itself
-    m_bitmapLockThickness->SetBitmap( KiScaledBitmap( locked_xpm, aFrame ) );
+    m_bitmapLockThickness->SetBitmap( KiScaledBitmap( BITMAPS::locked, aFrame ) );
 
     // Gives a minimal size of wxTextCtrl showing dimensions+units
     m_tcCTValue->SetMinSize( m_numericTextCtrlSize );