Add support for RequestTaskInventory capability

[SaladDais]

See https://feedback.secondlife.com/feature-requests/p/move-object-inventory-listing-to-http-capability

Apparently there's been a capability for fetching LLSD object inventory added recently, use it in the viewer to make object inventory loads faster. Make the viewer prefer to load inventory via the capability rather than over the deprecated Xfer system, though both are still supported. Inventories that took several minutes to load now load in 0.5 seconds or so.

I don't totally understand some of the existing object inventory code related to obj->mInventorySerialNum and obj->mExpectedInventorySerialNum mismatches, but this code seems to behave correctly, might deserve some extra scrutiny.

I'm not sure if release/maint-b is the correct base branch to use here, but that's the one that builds Linux correctly for me, so that's the one I use :)

[marchcat]

Thank you for the contribution!

The overall changes look good.
Please address the comments above and sync your branch with release/maint-b. Once done, I'll be able to merge.

[SaladDais]

Rebased on top of current release/maint-b and squashed in fixes to address review feedback