EXP-1221 FIXED Added a floater container class that provides an interface for...

EXP-1221 FIXED Added a floater container class that provides an interface for all former Side Tray panels.

The container implements a unified behavior similar to Side Tray tabs for all floaters:
- LLSD params are passed to the main panel (root panel containing other child panels) upon opening the floater via LLFloaterReg,
- LLSD params can be passed to one of the child panels using LLFloaterSidePanelContainer::openChildPanel()
- transient floaters (e.g. IM windows) should not hide when the container floater is clicked, so that it's possible to drag an inventory item from My Inventory floater to a docked IM window.