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.cpp

@@ -20,7 +20,15 @@ constexpr char KEY_VALUE_SEPARATOR_ESCAPE[] = "$0";
 constexpr char PARAM_SEPARATOR_ESCAPE[] = "$1";
 constexpr char ESCAPE_CHARACTER_ESCAPE[] = "$2";
 
+/// A placeholder for empty param packages to avoid empty strings
+/// (they may be recognized as "not set" by some frontend libraries like qt)
+constexpr char EMPTY_PLACEHOLDER[] = "[empty]";
+
 ParamPackage::ParamPackage(const std::string& serialized) {
+    if (serialized == EMPTY_PLACEHOLDER) {
+        return;
+    }
+
     std::vector<std::string> pairs;
     Common::SplitString(serialized, PARAM_SEPARATOR, pairs);
 
@@ -46,7 +54,7 @@ ParamPackage::ParamPackage(std::initializer_list<DataType::value_type> list) : d
 
 std::string ParamPackage::Serialize() const {
     if (data.empty())
-        return "";
+        return EMPTY_PLACEHOLDER;
 
     std::string result;
 
@@ -120,4 +128,12 @@ bool ParamPackage::Has(const std::string& key) const {
     return data.find(key) != data.end();
 }
 
+void ParamPackage::Erase(const std::string& key) {
+    data.erase(key);
+}
+
+void ParamPackage::Clear() {
+    data.clear();
+}
+
 } // namespace Common

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;