Set UnreadIMs icon to visible if messages were sent while offline.

MAINT-1891: FIX Updating the LLPipeline::clearRebuildGroups() to preserve HUD groups.  clearRebuildGroups() is called during the teleport process.  In the previous state, HUD objects were being incorrectly removed from the rebuild groups during the teleport process.  This change will preserve HUD groups and should resolve this issue.

CHUI-508 FIXED Attach to and Attach to HUD menus not showing in inventory floater to select attachment point

CHUI-512 FIXED (New incoming conversations take focus in message panel only and do not show toasts). Rejected update message when floater hasn't focus

CHUI-486: Post code review changes for last submit, just added in some parenthesis for conditional statements, thus making them more clear.

CHUI-486: Code cleanup, instead of using values such as 0, 1, 2 now using strings 'toast', 'flash', 'none'. These values are used whether to show a notification or not.

CHUI-447 FIXED Set session name as avatar's display name, if session is created by getting new message.

MAINT-1890: A hacky fix for flexi-prim updates after teleport by ignoring the sUpdateDelay value and simply updating all flexi-prims.  This change most likely reduces frame rate.

MAINT-1894: FIX This *should* fix a crash in the LLVolume::resizePath() method.  The callstacks indicated that the std library was throwing a length exception on the std::vector::resize() call.  Most likely cause was that the recent changes for flexi-prims were causing the mRenderRes to become negative.  And thus, the 1<<mRenderRes calculation was generating a really large value for num_render_sections.

CHUI-362 FIXED (Torn off conversation name is highlighted when selected in conversation list with different conversation showing in message panel): connect new method "returnFloaterToHost" to click on quasi-URL "Bring it back"

CHUI-486: Now the new chat preferences in drop down lists and checkboxes are storable. These values are accessible globally using gSavedSettings.

CHUI-517: Updating the do not disturb message automatically sent to other users when they attempt to contact a person in do not disturb mode.

CHUI-517: Altering the behavior and copy of the main menu status choices.  Busy has been replaced with Do Not Disturb.  And both status options have been switched to be indicated with a check rather than alternating text.

CHUI-362 WIP (Torn off conversation name is highlighted when selected in conversation list with different conversation showing in message panel): implemented the stub panel and a control of it's visibility

CHUI-362 WIP (Torn off conversation name is highlighted when selected in conversation list with different conversation showing in message panel): implemented method for a switch off tabs (switching to an invisible state)

CHUI-479 : WIP : Fixed the missing agent appearance in torn off dialogs by adding a consistency check and rebuild.

CHUI-450 : Fixed how the list of speakers is updated, always add the agent avatar in it, takes voice activation into account

CHUI-337 FIXED: To avoid confusion with a classes "...conversation..." and in accordance with the naming convention in the project, some classes and corresponding files should be renamed:
LLIMConversation -> LLFloaterIMSessionTab
LLIMFloater -> LLFloaterIMSession
LLNearbyChat -> LLFloaterIMNearbyChat
LLIMFloaterContainer -> LLFloaterIMContainer
LLNearbyChatBarListener -> LLFloaterIMNearbyChatListener
LLNearbyChatHandler -> LLFloaterIMNearbyChatHandler

MAINT-1228, MAINT-1668:  Deleting code that had been originally deleted to fix these issues, but then was accidentally re-introduced during a bad merge.