Make CssReloadHelper watch CSS new style on appearance change

If you have reload_style_on_change enabled, and use dynamic appearance
styling (style-dark.css and style-light.css), then changing the
appearance doesn't update the files that are being watched for reload.

Add a method to CssReloadHelper to change the CSS file being watched,
and also provide the CSS file as a parameter to the callback so setupCss
can be called on the right file when the file watcher triggers.

include/util/css_reload_helper.hpp

@@ -14,12 +14,14 @@ struct pollfd;
 namespace waybar {
 class CssReloadHelper {
  public:
-  CssReloadHelper(std::string cssFile, std::function<void()> callback);
+  CssReloadHelper(std::string cssFile, std::function<void(const std::string&)> callback);
 
   virtual ~CssReloadHelper() = default;
 
   virtual void monitorChanges();
 
+  virtual void changeCssFile(const std::string& newCssFile);
+
  protected:
   std::vector<std::string> parseImports(const std::string& cssFile);
 
@@ -42,7 +44,7 @@ class CssReloadHelper {
  private:
   std::string m_cssFile;
 
-  std::function<void()> m_callback;
+  std::function<void(const std::string&)> m_callback;
 
   std::vector<std::tuple<Glib::RefPtr<Gio::FileMonitor>>> m_fileMonitors;
 };