Use fixed font for source and instruction views

Code is traditionally viewed in fixed font, so the proportional font
used by default can be confusing.

libviews/instritem.cpp

@@ -22,6 +22,7 @@
 
 #include "instritem.h"
 
+#include <QFontDatabase>
 #include <QPixmap>
 #include <QPainter>
 #include <QPolygon>
@@ -87,6 +88,12 @@ InstrItem::InstrItem(InstrView* iv, QTreeWidget* parent,
     if (instr && (l = instr->line()))
         setText(7, l->name());
 
+    QFont font = QFontDatabase::systemFont(QFontDatabase::FixedFont);
+    setFont(0, font); // #
+    setFont(4, font); // hex
+    setFont(5, font); // mnenomic
+    setFont(6, font); // instruction args
+
     updateGroup();
     updateCost();
 }

libviews/sourceitem.cpp

@@ -22,6 +22,7 @@
 
 #include "sourceitem.h"
 
+#include <QFontDatabase>
 #include <QPixmap>
 #include <QRegExp>
 #include <QPainter>
@@ -65,6 +66,9 @@ SourceItem::SourceItem(SourceView* sv, QTreeWidget* parent,
     QString s = src;
     setText(4, s.replace( QRegExp(QStringLiteral("\t")), QStringLiteral("        ") ));
 
+    QFont font = QFontDatabase::systemFont(QFontDatabase::FixedFont);
+    setFont(4, font); // the source code line
+
     // to allow text selection after double click
     setFlags(flags() | Qt::ItemIsEditable);