Theme system

Solves #1

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

Depends on !21 (merged)

src/UI/LogConsole.cc

@@ -87,4 +87,6 @@ LogConsole::handleLogMessage(const std::string_view cat, const LogMessage &msg)
         label.append(c);
 
     addItem(label);
+    QListWidgetItem *lastItem = item(count() - 1);
+    scrollToItem(lastItem);
 }