Avoid performance penalty from type erasure

On average the affected code path is 0.1% faster and the result binary
is 10 KB smaller.

src/base/settingvalue.h

@@ -29,7 +29,6 @@
 #ifndef SETTINGVALUE_H
 #define SETTINGVALUE_H
 
-#include <functional>
 #include <type_traits>
 
 #include <QMetaEnum>
@@ -41,8 +40,6 @@
 template <typename T>
 class CachedSettingValue
 {
-    using ProxyFunc = std::function<T (const T&)>;
-
 public:
     explicit CachedSettingValue(const char *keyName, const T &defaultValue = T())
         : m_keyName(QLatin1String(keyName))
@@ -50,8 +47,11 @@ public:
     {
     }
 
+    // The signature of the ProxyFunc should be equivalent to the following:
+    // T proxyFunc(const T &a);
+    template <typename ProxyFunc>
     explicit CachedSettingValue(const char *keyName, const T &defaultValue
-            , const ProxyFunc &proxyFunc)
+                                , ProxyFunc proxyFunc)
         : m_keyName(QLatin1String(keyName))
         , m_value(proxyFunc(loadValue(defaultValue)))
     {