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

@@ -69,6 +69,23 @@ LogSink::closeLoggerClients() noexcept
                        [](const std::weak_ptr<Logger> &weakPtr) { return !weakPtr.expired(); }));
 }
 
+std::shared_ptr<Logger>
+LogSink::getLoggerClient(const std::string_view category) const
+{
+    auto end = std::end(clientLoggers);
+    auto it  = std::find_if(std::begin(clientLoggers), end,
+                           [category](const std::weak_ptr<Logger> &weakPtr) {
+                               std::shared_ptr<Logger> sharedPtr = weakPtr.lock();
+                               return sharedPtr && (sharedPtr->getCategoryView() == category);
+                           });
+    if (it == end)
+        throw std::logic_error("Logger was not found");
+    std::shared_ptr<Logger> sharedPtr = (*it).lock();
+    if (sharedPtr == nullptr)
+        throw std::logic_error("Logger has expired");
+    return sharedPtr;
+}
+
 LogMessage
 Logger::logEvent(const char *fileName, const char *functionName, const int lineNumber,
                  const LogLevel logSeverity) noexcept