import_gfx: Make file selection case-insensitive

This adds the upper-case version of files names to the selection box for
filesystems that support case-sensitive file names.

Fixes: lp:1832853
* https://bugs.launchpad.net/kicad/+bug/1832853

pcbnew/import_gfx/dxf_import_plugin.h

@@ -148,6 +148,7 @@ public:
     {
         wxArrayString list;
         list.Add( "dxf" );
+        list.Add( "DXF" );
         return list;
     }
 

pcbnew/import_gfx/graphics_import_plugin.h

@@ -62,7 +62,7 @@ public:
 
     /**
      * Return a list of wildcards that contains the file extensions
-     * handled by this plugin, separated with a coma.
+     * handled by this plugin, separated with a semi-colon.
      */
     wxString GetWildcards() const
     {
@@ -74,7 +74,7 @@ public:
             if( first )
                 first = false;
             else
-                ret += ", ";
+                ret += ";";
 
             ret += "*." + extension;
         }

pcbnew/import_gfx/svg_import_plugin.h

@@ -46,6 +46,7 @@ public:
     {
         wxArrayString list;
         list.Add( "svg" );
+        list.Add( "SVG" );
         return list;
     }