Rework UI lifecycle, finish renaming & cleanup

src/dusk/ui/document.cpp

@@ -101,6 +101,15 @@ bool Document::focus() {
     return false;
 }
 
+bool Document::has_focus() const {
+    if (mDocument == nullptr) {
+        return false;
+    }
+    auto* context = mDocument->GetContext();
+    const auto* focused = context != nullptr ? context->GetFocusElement() : nullptr;
+    return focused != nullptr && focused->GetOwnerDocument() == mDocument;
+}
+
 bool Document::set_document_styles(const Rml::String& rcss) {
     if (rcss.empty()) {
         mDocumentStyleSheets = nullptr;