Theme system

Solves #1

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

Depends on !21 (merged)

src/Lib/Log.cc

@@ -10,6 +10,18 @@ LogSinkDispatcher::getDispatcherName() const noexcept
 {
     return dispatcherName;
 }
+
+StderrLogSinkDispatcher::StderrLogSinkDispatcher(LogSink *const sink) noexcept
+    : LogSinkDispatcher(sink, std::string_view{ "stderr" })
+{
+}
+
+void
+StderrLogSinkDispatcher::handleLogMessage(const std::string_view category,
+                                          const LogMessage &msg) noexcept
+{
+    std::cerr << '[' << category << ']' << msg.getTextBuffer() << '\n';
+}
 }
 
 // Vivy::LogSink implementation