Sub-menu implementation in menu-panel

[fatih-erikli]

Fixes #1035.

[fatih-erikli]

Functionality must be done however the corners visually not looking well yet. @justvanrossum. I will work on it once we make sure it works correct.

[justvanrossum]

It works well, and looks quite ok. I would prefer the submenu-icon to look more like in macOS:

I don't immediately see a suitable unicode for it, although these come close:

But I think the "chevron-right" icon from https://tabler.io/icons is best: at least we wouldn't be depending on a system font's appearance.

My comment about a more dynamic items list (populated on-demand) applies here as well.

[justvanrossum]

Some more feedback regarding appearance and behavior.

• The red selected items should align: the submenu should be a bit higher

• If I look at macOS, the "parent" items gets a different (gray) highlight when you show a sub-sub-menu:

• When you go back to the parent-parent, I think the sub-sub-menu should hide again:

[fatih-erikli]

This is not ready yet, I will rethink the storing open menu instances, currently I read them in the DOM layout. The main PR is ready for review.