Browser: Simplify the History class and fix back/forward history push

This code was previously relying on the PageView::on_load_start hook
firing synchronously when calling PageView::load(). This was not
happening with WebContentView, so it broke the back/forward history.

Instead, we now differentiate between history navigations and normal
loads in Tab::load(). History navigations don't push new entries into
history, but instead just move the history pointer.

Applications/Browser/CMakeLists.txt

@@ -3,6 +3,7 @@ set(SOURCES
     BrowserConsoleClient.cpp
     ConsoleWidget.cpp
     DownloadWidget.cpp
+    History.cpp
     InspectorWidget.cpp
     main.cpp
     Tab.cpp