Theme system

Solves #1

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

Depends on !21 (merged)

src/UI/AboutWindow.cc

@@ -55,6 +55,7 @@ AboutWindow::LicenceLabel::LicenceLabel(QWidget *parent, const QString &url,
                                         const Qt::TextFormat format)
     : QTextEdit(parent)
 {
+    VIVY_LOG_CTOR() << "Creating label for licence '" << url << '\'';
     QFile content(url);
     if (!content.open(QIODevice::ReadOnly | QIODevice::Text))
         throw std::runtime_error("Failed to open file that should be accessible");
@@ -83,6 +84,7 @@ AboutWindow::AboutWindow(QWidget *parent) noexcept
     : QMainWindow(parent, Qt::Dialog)
     , panels(new QTabWidget)
 {
+    VIVY_LOG_CTOR() << "Creating the 'Vivy - About' window";
     setWindowIcon(QIcon(VIVY_ICON_APP));
     setWindowTitle("Vivy - About");