[Fonts] Make the editor's fallback and Reset preview-independent

- Add ThemeManager::defaultAppFont(), which resolves an unset role against the
  startup font instead of currentFonts, and use it in FontGridWidget::loadFonts
  so the grid never shows the live preview as the theme's default.
- Reload the live theme in PaletteEditorDialog::onReset before re-reading the
  grid, so Reset restores the saved fonts before loadFonts resolves fallbacks.

cockatrice/src/interface/theme_manager.h

@@ -152,6 +152,11 @@ public:
     /** @brief Resolves an application font role: the theme's stored [Fonts]
      *         value when present, otherwise the application font. */
     QFont appFont(AppFont::Role role) const;
+    /** @brief Resolves an application font role against the startup font only,
+     *         ignoring any previewed config in currentFonts. Editors that show
+     *         "what this role resolves to" must use this, because appFont()
+     *         reflects a live preview while the editor is open. */
+    QFont defaultAppFont(AppFont::Role role) const;
 
     QBrush &getBgBrush(Role zone);
     QBrush getExtraBgBrush(Role zone, int zoneId = 0);