feat: Use monospace as default font; monospace only in preferences

1. Use monospace as the default font.

2. Now only monospace fonts will be displayed in the font selection
   window.

This suggestion is from @arpit in Telegram group.

> One suggestion please keep Monospace or any other font which provides
> good indentation. Even in my college many students deleted your
> editorjust because of that formatting problem because of default Sans
> Serif font.

src/Core/SettingsManager.cpp

@@ -160,7 +160,7 @@ QString SettingManager::getTemplatePathPython()
 }
 QString SettingManager::getFont()
 {
-    return mSettings->value("font", "").toString();
+    return mSettings->value("font", "monospace").toString();
 }
 
 QRect SettingManager::getGeometry()

src/preferencewindow.cpp

@@ -257,7 +257,7 @@ void PreferenceWindow::on_hotkeys_clicked(bool checked)
 void PreferenceWindow::on_font_button_clicked()
 {
     bool ok = false;
-    QFont fp = QFontDialog::getFont(&ok, currentFont);
+    QFont fp = QFontDialog::getFont(&ok, currentFont, this, "Choose a font", QFontDialog::MonospacedFonts);
 
     if (ok)
     {