Fix some coverity warnings (not initialized members). Add comments. Better (more informative) title in dialog to download .pretty libs.

common/class_bitmap_base.cpp

@@ -58,6 +58,7 @@ BITMAP_BASE::BITMAP_BASE( const wxPoint& pos )
 BITMAP_BASE::BITMAP_BASE( const BITMAP_BASE& aSchBitmap )
 {
     m_Scale = aSchBitmap.m_Scale;
+    m_ppi   = aSchBitmap.m_ppi;
     m_pixelScaleFactor = aSchBitmap.m_pixelScaleFactor;
     m_image = new wxImage( *aSchBitmap.m_image );
     m_bitmap = new wxBitmap( *m_image );