Theme system

Solves #1

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

Depends on !21 (merged)

src/UI/Theme/Theme.cc

@@ -147,18 +147,9 @@ Theme::fromSettings(const QString &id, QSettings *const settings) noexcept
 Theme *
 Theme::fromVivyTheme(const QString &id, const VivyTheme which) noexcept
 {
-    Theme *ret        = new Theme(id);
-    ret->d->classType = Type::QssFile;
-    switch (which) {
-    case VivyTheme::Dark:
-        ret->d->fileName = QStringLiteral(":qdarkstyle/dark/style.qss");
-        return ret;
-    case VivyTheme::Light:
-        ret->d->fileName = QStringLiteral(":qdarkstyle/light/style.qss");
-        return ret;
-    }
-
-    qFatal("Unknown VivyTheme!");
+    return fromQssFile(id, which == VivyTheme::Dark
+                               ? QStringLiteral(":qdarkstyle/dark/style.qss")
+                               : QStringLiteral(":qdarkstyle/light/style.qss"));
 }
 
 void