Warning for losing unsaved work in process modeler doesn't always show

**Describe the bug**
Due to the nature of navigation (using the "#"), the `onbeforeunload` hook to warn users about abandoning unsaved work does not work when navigating via buttons or hyperlinks inside the modeler. If you use regular, in-modeler navigation, like eg. closing the model or clicking on one of the links to other designers (like the Forms designer), you don't get a warning and lose your work. Going back to the process model just shows you the last saved version again and all work is lost. You do get the warning when you go to another site (from the address bar or a bookmark) or do a full refresh of the page but the hash-navigation ("the single page app way") doesn't trigger the hook, thus no warning, thus possibly losing unsaved work. The old flowable-explorer didn't use this manner of navigation so this problem didn't occur there.

**Expected behavior**
So whenever you edit a model and make the slightest change, the onbeforeunload hook should be added but also a hook to the hash-navigation-framework that is used to display a custom warning prompt. If not possible I guess Flowable should shelve the changes (eg. on localstorage) and gives the possibility to start working from those shelved changes instead of the ones saved in the DB.

**Additional context**
Flowable 6.5.0.6

プロセスモデラーの onbeforeunload 処理とハッシュナビゲーションフレームワークから始め、編集を行った後に、モデラー内のナビゲーションと他のデザイナーへのリンクを通じたナビゲーションを再現します。完了条件は、内部ナビゲーションで未保存の変更に対するカスタム警告が表示され、変更が破棄されないことです。同時に、外部ナビゲーションと更新に対する既存の警告も引き続き機能する必要があります。

索引モデルが issue の本文から書いたものです。