EXP-1742 FIX -- Clicking IM notification or receiving multiple IM notifications in quick succession crashes to desktop

* Moved toast logic for mouse hover out of the draw call to avoid chain of callbacks that lead to reordering of the draw list while we are iterating over it.

The error sometimes led to the following issues with maximized viewer window on MS Windows:
* displaying incorrect resolution
* failure to update snapshot

Reviewed by Richard.

force all floaters to have follows flags = FOLLOWS_NONE and use llfloaterview::reshape logic instead
removed existing follows flags from any floater XUI and deprecated the follows parameter for floaters