Load custom Qt themes from yuzu data directory

- Directory is qt_themes, each theme must be in one folder
    - It should contain a file "style.qss"
    - It may contain an "icons" sub-directory, to override included icons
      (with files like mytheme/icons/colorful/48x48/star.png for example)
    - Directories ending by "_dark" are reserved for dark variant icons.
      They are not listed as themes in the UI.
- If theme directory contains "dark" or "midnight", theme will be considered dark

src/suyu/main.h

@@ -165,6 +165,12 @@ class GMainWindow : public QMainWindow {
         CREATE_SHORTCUT_MSGBOX_APPVOLATILE_WARNING,
     };
 
+    /**
+     * Try to load a stylesheet from its path. If the path starts with ":/", its embedded in the app
+     * @returns true if the text file could be opened as read-only
+     */
+    bool tryLoadStylesheet(const QString& theme_path);
+
 public:
     void filterBarSetChecked(bool state);
     void UpdateUITheme();