STORM-36 FIXED As a User, I want to control how long a chat toast appears...

STORM-36 FIXED As a User, I want to control how long a chat toast appears before it fades. Please add fade time back to Chat preferences.

- Added two spinners to the Chat preferences tab that control NearbyToastLifeTime and NearbyToastFadingTime
- Added callbacks to the LLNearbyChatScreenChannel that update these properties if they were changed

Refactoring of LLToast:
- Removed code that was making toast transparent from LLToast::draw()
- Modified LLToast interface that relates to showing and hiding toast, so that all screen channels can use LLToast universally.
- Replaced in LLScreenChannel calling methods of old interface to new ones.