OSD / Imgui: Add font size spinbox to Interface Pane.

Adjust some imgui windows to work better with different font sizes.

Source/Core/DolphinQt/Settings/InterfacePane.h

@@ -6,6 +6,7 @@
 #include <QWidget>
 
 class ConfigBool;
+class ConfigInteger;
 class ConfigRadioInt;
 class ConfigStringChoice;
 class QLabel;
@@ -54,6 +55,7 @@ private:
   ConfigBool* m_checkbox_confirm_on_stop;
   ConfigBool* m_checkbox_use_panic_handlers;
   ConfigBool* m_checkbox_enable_osd;
+  ConfigInteger* m_osd_font_size;
   ConfigBool* m_checkbox_show_active_title;
   ConfigBool* m_checkbox_pause_on_focus_lost;
   ConfigRadioInt* m_radio_cursor_visible_movement;