main: Add resolution scale label in the status bar

Shows the resolution scale as "Scale: {}x" in the status bar, where {} is a floating point value representing the current resolution scaling factor.

src/yuzu/main.h

@@ -328,6 +328,7 @@ private:
     // Status bar elements
     QLabel* message_label = nullptr;
     QLabel* shader_building_label = nullptr;
+    QLabel* res_scale_label = nullptr;
     QLabel* emu_speed_label = nullptr;
     QLabel* game_fps_label = nullptr;
     QLabel* emu_frametime_label = nullptr;