Fix a few wording issues in dialogs (dialog plot and dialog move exact) for translators (more explicit words/sentences to remove ambiguities).

Pcbnew: Plot files: for copper layers, create suffix from user copper layer names instead of default copper layer names, like in last stable Kicad version (bzr 4022), to build the plot files filenames.
Better test of illegal chars in plot filename, both in plot dialog and functions aclled by Python scripts.
Fix a Plotcontroller issue to make SetUseGerberExtensions() work as expected from Python scripts ( from  Henner Zeller'patch with a fix to be sure it works properly from a Python script)
Note also using Protel Gerber Extensions is now a bad practice, because the official Gerber extension is .gbr since X2 Gerber version. But some users still use them)

pcbnew/pcbplot.cpp

@@ -45,7 +45,7 @@
 #include <build_version.h>
 
 
-const wxString GetGerberExtension( LAYER_NUM aLayer )
+const wxString GetGerberProtelExtension( LAYER_NUM aLayer )
 {
     if( IsCopperLayer( aLayer ) )
     {
@@ -286,28 +286,32 @@ void AddGerberX2Attribute( PLOTTER * aPlotter,
 }
 
 
-void BuildPlotFileName( wxFileName*     aFilename,
-                        const wxString& aOutputDir,
-                        const wxString& aSuffix,
-                        const wxString& aExtension )
+void BuildPlotFileName( wxFileName* aFilename, const wxString& aOutputDir,
+                        const wxString& aSuffix, const wxString& aExtension )
 {
+    // aFilename contains the base filename only (without path and extension)
+    // when calling this function.
+    // It is expected to be a valid filename (this is usually the board filename)
     aFilename->SetPath( aOutputDir );
 
     // Set the file extension
     aFilename->SetExt( aExtension );
 
-    /* remove leading and trailing spaces if any from the suffix, if
-       something survives add it to the name;
-       also the suffix can contain some not allowed chars in filename (/ \ .),
-       so change them to underscore
-    */
+    // remove leading and trailing spaces if any from the suffix, if
+    // something survives add it to the name;
+    // also the suffix can contain some not allowed chars in filename (/ \ . :),
+    // so change them to underscore
+    // Remember it can be called from a python script, so the illegal chars
+    // have to be filtered here.
     wxString suffix = aSuffix;
     suffix.Trim( true );
     suffix.Trim( false );
 
-    suffix.Replace( wxT("."), wxT("_") );
-    suffix.Replace( wxT("/"), wxT("_") );
-    suffix.Replace( wxT("\\"), wxT("_") );
+    wxString badchars = wxFileName::GetForbiddenChars(wxPATH_DOS);
+    badchars.Append( '%' );
+
+    for( unsigned ii = 0; ii < badchars.Len(); ii++ )
+        suffix.Replace( badchars[ii], wxT("_") );
 
     if( !suffix.IsEmpty() )
         aFilename->SetName( aFilename->GetName() + wxT( "-" ) + suffix );
@@ -368,9 +372,18 @@ bool PLOT_CONTROLLER::OpenPlotfile( const wxString &aSuffix,
     if( EnsureFileDirectoryExists( &outputDir, boardFilename ) )
     {
         wxFileName fn( boardFilename );
-        BuildPlotFileName( &fn, outputDirName, aSuffix, GetDefaultPlotExtension( aFormat ) );
+        wxString fileExt = GetDefaultPlotExtension( aFormat );
 
-        m_plotter = StartPlotBoard( m_board, &GetPlotOptions(), ToLAYER_ID( GetLayer() ), fn.GetFullPath(), aSheetDesc );
+        // Gerber format can use specific file ext, depending on layers
+        // (now not a good practice, because the official file ext is .gbr)
+        if( GetPlotOptions().GetFormat() == PLOT_FORMAT_GERBER &&
+            GetPlotOptions().GetUseGerberExtensions() )
+            fileExt = GetGerberProtelExtension( GetLayer() );
+
+        BuildPlotFileName( &fn, outputDirName, aSuffix, fileExt );
+
+        m_plotter = StartPlotBoard( m_board, &GetPlotOptions(), ToLAYER_ID( GetLayer() ),
+                                    fn.GetFullPath(), aSheetDesc );
     }
 
     return( m_plotter != NULL );