citra_qt/configuration: misc input tab improvements

* Added a context menu on the buttons including Clear & Restore Default

* Allow clearing (unsetting) inputs. Added a Clear All button

* Allow restoring a single input to default (instead of all)

src/citra_qt/configuration/configure_input.h

@@ -73,6 +73,9 @@ private:
     void loadConfiguration();
     /// Restore all buttons to their default values.
     void restoreDefaults();
+    /// Clear all input configuration
+    void ClearAll();
+
     /// Update UI to reflect current configuration.
     void updateButtonLabels();