SelectableTextEditorWidget: Handle display settings

Avoid code repetition inside UnifiedDiffEditorWidget
and SideDiffEditorWidget.

Connect to display settings changed signal and apply
all but 4 settings (which are not desired for diff editor).

Change-Id: If4ae7f7c55690dac04fd1a00f8822db666a9ce2b
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/diffeditor/selectabletexteditorwidget.h

@@ -5,6 +5,8 @@
 
 #include <texteditor/texteditor.h>
 
+namespace TextEditor { class DisplaySettings; }
+
 namespace DiffEditor {
 namespace Internal {
 
@@ -29,6 +31,8 @@ public:
     ~SelectableTextEditorWidget() override;
     void setSelections(const DiffSelections &selections);
 
+    void setDisplaySettings(const TextEditor::DisplaySettings &displaySettings) override;
+
     static DiffSelections polishedSelections(const DiffSelections &selections);
     static void setFoldingIndent(const QTextBlock &block, int indent);