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

@@ -2,12 +2,32 @@
 
 using namespace Vivy;
 
+LogSink::~LogSink() noexcept {}
+
 LogMessage::~LogMessage() noexcept { parentLogger->sendLogMessage(*this); }
-Logger::~Logger() noexcept { parentLogSink->closeLoggerClient(this); }
+
+std::shared_ptr<LogSink>
+LogSink::newSink() noexcept
+{
+    struct makeSharedEnabler : public LogSink {
+        // NOTE: For make_shared with private CTor
+    };
+    return std::make_shared<makeSharedEnabler>();
+}
 
 void
 LogSink::closeLoggerClient(Logger *const logger) noexcept
 {
+    clientLoggers.erase(std::remove_if(std::begin(clientLoggers), std::end(clientLoggers),
+                                       [logger](const std::shared_ptr<Logger> &ptr) {
+                                           return ptr.get() == logger;
+                                       }));
+}
+
+void
+LogSink::closeLoggerClient(const std::shared_ptr<Logger> &ptr) noexcept
+{
+    closeLoggerClient(ptr.get());
 }
 
 LogMessage