citra-qt: Add base support for hotkey reconfiguration + UI (whole of PR citra-emu/citra#3786)

* Adds a new Hotkeys tab in the Controls group.
* Right click to reconfigure.
* See the original PR for more details & screenshots.

src/citra_qt/configuration/configure_general.h

@@ -20,7 +20,6 @@ public:
     explicit ConfigureGeneral(QWidget* parent = nullptr);
     ~ConfigureGeneral() override;
 
-    void PopulateHotkeyList(const HotkeyRegistry& registry);
     void ResetDefaults();
     void applyConfiguration();
     void retranslateUi();