Solder mask clearance default value: set default to 0, taking advice from

both Ucamco and Eurocircuits.
Update message in plot dialog about best solder mask clearance value.
Show warning in plot dialog if these values are no 0.

pcbnew/dialogs/dialog_plot.cpp

@@ -396,6 +396,18 @@ void DIALOG_PLOT::SetPlotFormat( wxCommandEvent& event )
     // this option exist only in DXF format:
     m_DXF_plotModeOpt->Enable( getPlotFormat() == PLOT_FORMAT::DXF );
 
+    // The alert message about non 0 solder mask min width and margin is shown
+    // only in gerber format and if min mask width or mask margin is not 0
+    BOARD* board = m_parent->GetBoard();
+    const BOARD_DESIGN_SETTINGS& brd_settings = board->GetDesignSettings();
+
+    if( getPlotFormat() == PLOT_FORMAT::GERBER &&
+        ( brd_settings.m_SolderMaskMargin || brd_settings.m_SolderMaskMinWidth ) )
+        m_PlotOptionsSizer->Show( m_SizerSolderMaskAlert );
+    else
+        m_PlotOptionsSizer->Hide( m_SizerSolderMaskAlert );
+
+
     switch( getPlotFormat() )
     {
     case PLOT_FORMAT::SVG: