Restore correct font after the Cell Edit dock is set to plain text

QScintilla seems to reset many settings when setLexer is called, so we
need to restore the font when switching from the lexer modes to the mode
without lexer (plain text).

See issue #1793.

src/docktextedit.cpp

@@ -36,9 +36,9 @@ void DockTextEdit::reloadSettings()
     reloadLexerSettings(xmlLexer);
 
     // Set the databrowser font for the plain text editor.
-    QFont textFont(Settings::getValue("databrowser", "font").toString());
-    textFont.setPointSize(Settings::getValue("databrowser", "fontsize").toInt());
-    setFont(textFont);
+    plainTextFont = QFont(Settings::getValue("databrowser", "font").toString());
+    plainTextFont.setPointSize(Settings::getValue("databrowser", "fontsize").toInt());
+    setFont(plainTextFont);
 
     setupSyntaxHighlightingFormat(jsonLexer, "comment", QsciLexerJSON::CommentLine);
     setupSyntaxHighlightingFormat(jsonLexer, "comment", QsciLexerJSON::CommentBlock);
@@ -84,6 +84,8 @@ void DockTextEdit::setLanguage(Language lang)
     case PlainText: {
         setLexer(nullptr);
         setFolding(QsciScintilla::NoFoldStyle);
+        // This appears to be reset by setLexer
+        setFont(plainTextFont);
         break;
     }
     case JSON:

src/docktextedit.h

@@ -42,6 +42,7 @@ protected:
     static QsciLexerXML* xmlLexer;
 private:
     Language m_language;
+    QFont plainTextFont;
 };
 
 #endif