STORM-1093 FIX "Dock" icon is still shown after a side panel has been docked with Ctrl+Shift+W.

Reason:
When an undocked side tray tab floater got closed with Ctrl+Shift+W,
LLSideTray::setTabDocked() was called. It docked the floater but didn't update
the dock/undock icon.

Fix:
Made setTabDocked() a general purpose method, not a hack suitable for using
only as a floater close callback in the basic viewer mode.
It now updates the dock/undock icon.

Other changes:
* Replaced numerous calls to toggleTabDocked with setDocked(),
  that is safer because does exactly what you want.
* Got rid of a duplicated floater close callback.