MainWindow: renamed class to match new style

Renamed mainWindow to MainWindow and engine::getMainWindow() to
engine::mainWindow(). Additionally reduced some header dependencies.

src/gui/LfoControllerDialog.cpp

@@ -35,7 +35,7 @@
 #include "embed.h"
 #include "engine.h"
 #include "led_checkbox.h"
-#include "main_window.h"
+#include "MainWindow.h"
 #include "tooltip.h"