[FIRE-34881] Library toggle

[TlNKER]

Fixes important part* of FIRE-34881: https://jira.firestormviewer.org/browse/FIRE-34881

Allows user to go into cog wheel > Tabs & Views > Show Library. This toggles whether Library shows in the inventory floater or not. The underpinning objects are still fully accessible to the system, only hidden from the user's view, confirmed that objects from Library still works (Avatar > Avatar Welcome Pack... and all aspects of LSL Bridge works with the Library hidden).

• New debug setting FSShowLibraryFolder (default: shown), persisted per-user
• New LLInventoryPanel::setLibraryFolderVisible() builds or tears down just the Library root's folder view live, without a restart. The underlying inventory model/data for Library is untouched either way, since other systems (default attachments, outfit lookups, etc.) depend on being able to resolve the Library folder ID regardless of whether its view is shown.
• Newly built folder views need an explicit arrangeAll() to actually become visible (LLFolderViewFolder::addFolder() has its own requestArrange() call commented out), matching the existing pattern used in openAllFolders()/closeAllFolders()/setSorter().
• New "Show Library" checkbox item added to the gear menu, wired through the existing Inventory.GearDefault.Custom.Action/Check callbacks.

All changes are tagged <FS:TP> per CONTRIBUTING.md, with the original code preserved as comments where replaced as instructed.

(I've decided that was the important part as I was the one raising the ticket to begin with :p)

[Hecklezz]

All looks good, however this also hides the Library folder from the texture picker when editing objects, the load sky and water picker in the day/night cycle floater, and the my environments floater under Top Menu->World->Environments->My Environments

Is this intentional of this new toggle, or isn't?

[TlNKER]

That is by design. I figured black/white would work best. Either you have it or you don't. (obviously without impacting or effecting viewer performance, so the viewer can still call on the elements in the Library, but it wont show to the user).