FlatMenuUI: remove JMenuBar dependency

[rogerbj]

A ClassCastException is thrown in FlatMenuUI.java when using a CommandMenuBar.java in JIDE. This occurs since FlatMenuUI assumes there is always a JMenuBar as parent for a JMenuItem. The problem occurs if resizing the width of the window whereas JIDE automatically replaces JMenu's that doesn't fit with a clickable icon. Clicking it shows a popup window with the menus that doesn't fit to display. The owner of the JMenuItem's is then a CommandBarPane instance rather than the original CommandMenuBar. The CCE happens when clicking the icon.

This PR generalizes FlatMenuUI to not require a JMenuBar.

With this change it now works fine with JIDE.

[DevCharly]

Thanks for the PR.

I wonder in which situation the ClassCastException is thrown?

CommandMenuBar extends JMenuBar.
So as long as the JideMenu is a direct child of the CommandMenuBar, it should work fine.

[rogerbj]

@DevCharly I've updated the original PR description with a sample screenshot that uses this PR branch.

[DevCharly]

Thx. Finally I understand why JideMenu overrides isTopLevelMenu() 😄

Instead of merging this PR, I've cherry picked your commit 4d4b90c.
It is now in FlatLaf/main in commit 0c00117.

This PR contains a lot of "Merge remote-tracking branch 'origin/master'" commits,
which I did not like to add to the history of this repo.

For future PRs, please base them on main branch of this repo.

To update dbvis/FlatLaf repo, it would be better to avoid merging from this repo.
Better fetch commits from this repo to a local FlatLaf repo, do a local fast-forward merge and then push to dbvis/FlatLaf repo.

[rogerbj]

@DevCharly Super! Yeah, sorry the PR did include a lot of commits. Will be more careful in the future.