Gerbview: rename diff mode by forced opacity display mode and make opacity adjustable.

The diff mode (option in left toolbar) was never a differential display mode.
It is actually an option to force the alpha channel (opacity) of layer colors
to a given value.
So change the name and tool tips to force opacity mode, and make the opacity
parameter adjustable in preferences.

Gerbview: cleanup: remove a old and dead code.

gerbview/gerbview_settings.cpp

@@ -56,6 +56,9 @@ GERBVIEW_SETTINGS::GERBVIEW_SETTINGS() :
     m_params.emplace_back( new PARAM<bool>( "appearance.show_page_limit",
             &m_Display.m_DisplayPageLimits, false ) );
 
+    m_params.emplace_back( new PARAM<double>( "appearance.mode_opacity_value",
+            &m_Display.m_OpacityModeAlphaValue, 0.6 ) );
+
     m_params.emplace_back( new PARAM_LIST<wxString>( "system.drill_file_history",
             &m_DrillFileHistory, {} ) );