Ensure feature parity with other AppWindows for vault options

[JaniruTEC]

This PR ensures feature parity between the other methods in FxApplicationWindows and the new #showVaultOptionsWindow() added by #2985. Therefore this PR depends on #2985.

"Feature parity" in this case consists of:

• allowing the caller to provide their own parent stage instead of using the primaryStage provided by FxApplicationWindows (4a191ac)
• checking the provided stage and making it visible if it was not previously visible (0ce77cb)

Also see: #2981

[infeo]

@JaniruTEC Can you merge develop into this branch? Maybe just the UI is buggy, but even after merging its parent branch the Github UI is showing a diff.

[JaniruTEC]

Merging fixed it for me. I also noticed the additional diffs.

[infeo]

I'm not convinced of this change. We enforce showing the main window, but in the next step we allow to have an arbitrary owner, not necessarily related to the main window. I would vote to remove showMainWindow() and just use the owner, in the sense of var actualowner = Optional.ofNullable(owner).elseGet(mainWindow). But looking at the functions of showUnlockWorkflow(), we also check if the vault is locked/unlocked. So overall, there is something missing here.

In the end, we want ensure that the user flow is sequential. First do something in the options, then close the options and only then unlock. If it is unlocked, do not allow opening the vault options.

By showing the mainWindow before opening the vault options, we ensure this. If we allow now arbitrary owner, unrelated owner, we could loose such sequentiality. I would say we apply YAGNI for now.