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.

test/utils/css_reload_helper.cpp

@@ -10,7 +10,7 @@
 
 class CssReloadHelperTest : public waybar::CssReloadHelper {
  public:
-  CssReloadHelperTest() : CssReloadHelper("/tmp/waybar_test.css", [this]() { callback(); }) {}
+  CssReloadHelperTest() : CssReloadHelper("/tmp/waybar_test.css", [this](const std::string&) { callback(); }) {}
 
   void callback() { m_callbackCounter++; }