Unregister window observer before theme manager is deallocated

Technically this probably isn't actually necessary since ThemeManager looks like it should remain allocated until the program quits, but...

Signed-off-by: Kenneth Chew <79120643+kthchew@users.noreply.github.com>

launcher/ui/themes/ThemeManager.cpp

@@ -50,6 +50,11 @@ ThemeManager::ThemeManager()
     initializeCatPacks();
 }
 
+ThemeManager::~ThemeManager()
+{
+    stopSettingNewWindowColorsOnMac();
+}
+
 /// @brief Adds the Theme to the list of themes
 /// @param theme The Theme to add
 /// @return Theme ID
@@ -179,6 +184,8 @@ void ThemeManager::setTitlebarColorOnMac(WId windowId, QColor color)
 {}
 void ThemeManager::setTitlebarColorOfAllWindowsOnMac(QColor color)
 {}
+void ThemeManager::stopSettingNewWindowColorsOnMac()
+{}
 #endif
 
 QList<IconTheme*> ThemeManager::getValidIconThemes()