Add option to hide title bar in Zen Mode (even outside full screen mode)

[W4RH4WK]

This PR is related to #33607.

This PR fixes #115570.

Please revisit the config option description. I don't know whether my wording is appropriate here.

[ghost]

All CLA requirements met.

[W4RH4WK]

Realized that with this implementation the menu bar cannot be toggled correctly anymore. Will try to fix and re-open this.

[W4RH4WK]

After investigating the menu bar issue, I don't think this can be implemented fully without adding additional flags to other components.

It works fine when window.menuBarVisibility set to toggle, but currently not for `classic. This happens because the menu bar focus / unfocus logic cannot query whether Zen Mode is currently active or not.

vscode/src/vs/base/browser/ui/menu/menubar.ts

Line 749 in 11cd760

} else if (this.options.visibility === 'classic' && browser.isFullscreen()) {

Furthermore, during debugging I discovered that some functions in layout.ts related to the menu bar might be dead code, namely getMenubarVisibility and toggleMenuBar.