MAINT-922 FIX Client side code to catch the SCRIPT_PERMISSION_TELEPORT request from similator and display a notification
Reviewed by Simon

Incoming changes from two different project repos left viewer-development with
LLInventoryModel::removeItem() and removeCategory() plus free functions
remove_item() and remove_category() in llinventoryfunctions.cpp.
remove_category() was actually the better implementation; migrated its body
into LLInventoryModel::removeCategory(). Clearly the previous state of affairs
-- with LLInventoryModel::removeItem() plus a remove_category() free function
in a very different source file -- fooled two different developers into
overlooking the other of the pair. Unfortunately we each added different
"missing" functions, leaving us with a complete set of four.
Fix existing references to remove_item() and remove_category() free functions.

Notification handlers now use LLHandles to screen channels.

Fixed boost trackable objects usage.

LL_PATH_USER_SETTINGS. This will allow a common write location for development
and installed configurations. Should correct installed configuration from
permission failures on windows systems where writing to the install directory
requires elevation of rights to administrator.

Subtask of STORM-637

As far as I can tell, if login fails with the "Region has begun the logout process" message, gAgentAvatarp gets destroyed, which leads to the crash.
Adding a check that should prevent it (could not reproduce to make sure).

Introduce new LLInventoryModel::removeCategory() method comparable to
removeItem(), but for folder objects (using changeCategoryParent() rather than
changeItemParent()). Introduce removeObject() method that calls one of the
above, depending on runtime object type.

The body of removeItem() was already nearly identical to the body of
changeItemParent(); this eliminates the redundancy.
Improve logging for changeItemParent().

It's not really clear why they're in llinventoryfunctions.{h,cpp} instead of
LLInventoryModel, and in fact LLInventoryModel::removeItem() already contains
code essentially cloned from change_item_parent() -- or perhaps vice-versa.
This changeset addresses only migrating the functions, and fixing up existing
references, to simplify code review.