Add context menu entry to toggle offline tabs

[Nerixyz]

Adds an entry to toggle the visibility of offline tabs, as mentioned in #5315 (comment).

I'll note that the context menus for the notebook, split-header and the menu of a split are shown when pressing the mouse, not when releasing it, which is unintuitive.

[pajlada]

I'll note that the context menus for the notebook, split-header and the menu of a split are shown when pressing the mouse, not when releasing it, which is unintuitive.

This is a platform-specific discrepancy we have. On Linux, the norm is to show the menu on mouse-down rather than mouse-up

[pajlada]

LGTM - there's some UX improvements that could be done with tab visibility in general but that doesn't have to be completed in this PR, or in this release. This will already help reduce support traffic of "oops all my tabs are gone"