DolphinQt2: replace icons with labels in controllers dialog

Icons without labels are bad for usability, so bring this back in line
with how DolphinWX does it.

Some reading:

- https://www.nngroup.com/articles/icon-usability/
  > Summary: A user’s understanding of an icon is based on previous
  > experience. Due to the absence of a standard usage for most icons,
  > text labels are necessary to communicate the meaning and reduce
  > ambiguity.
- http://uxmyths.com/post/715009009/myth-icons-enhance-usability
- http://edwardsanchez.me/blog/13589712
- https://uxdesign.cc/do-icons-need-labels-6cb4f4282c00

Source/Core/DolphinQt2/Config/ControllersWindow.h

@@ -47,11 +47,6 @@ private:
   void ConnectWidgets();
   void LoadSettings();
 
-  // Icons
-  QString m_configure_icon;
-  QString m_gamecube_icon;
-  QString m_wii_icon;
-
   // Main
   QVBoxLayout* m_main_layout;
   QDialogButtonBox* m_button_box;
@@ -59,7 +54,6 @@ private:
   // Gamecube
   std::array<MappingWindow*, 4> m_gc_mappings;
   QGroupBox* m_gc_box;
-  QLabel* m_gc_label;
   QFormLayout* m_gc_layout;
   std::array<QComboBox*, 4> m_gc_controller_boxes;
   std::array<QPushButton*, 4> m_gc_buttons;
@@ -68,7 +62,6 @@ private:
   // Wii Remote
   std::array<MappingWindow*, 4> m_wiimote_mappings;
   QGroupBox* m_wiimote_box;
-  QLabel* m_wii_label;
   QFormLayout* m_wiimote_layout;
   std::array<QLabel*, 4> m_wiimote_labels;
   std::array<QComboBox*, 4> m_wiimote_boxes;