Theme system

Solves #1

Can choose between QtCreator's themes and base Vivy theme, or system's theme.

Depends on !21 (merged)

src/Lib/Utils.hh

@@ -26,6 +26,7 @@
 
 // Use chrono instead of std::chrono...
 namespace chrono = std::chrono;
+using chrono::duration_cast;
 
 // Prety define for OpenMP's parallel for loop with indentation not fucked up
 // by clang-format.
@@ -58,8 +59,23 @@ concept PropertyConstViewable = requires(T element)
     // clang-format on
 };
 
+// Concept with a better name for inheritence between types
 template <class T, class U>
 concept Derived = std::is_base_of<U, T>::value;
+
+// Concept for string things
+template <typename T>
+concept StringType = requires(T str)
+{
+    // clang-format off
+    typename T::iterator;   // Can iterate
+    typename T::value_type; // The `Char`
+    { str.size() }  -> std::convertible_to<std::size_t>;
+    { str.begin() } -> std::same_as<typename T::iterator>;
+    { str.end() }   -> std::same_as<typename T::iterator>;
+    { str[0] }      -> std::same_as<typename T::value_type>;
+    // clang-format on
+};
 }
 
 namespace Vivy::Utils