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/common/param_package.h

@@ -32,6 +32,8 @@ public:
     void Set(const std::string& key, int value);
     void Set(const std::string& key, float value);
     bool Has(const std::string& key) const;
+    void Erase(const std::string& key);
+    void Clear();
 
 private:
     DataType data;