Bugfix/focusframe

FocusFrame removes unit when it gets destroyed so you'll never have a target you can't unfocus.
FocusFrame remember target on reloadUI.

Fixes #106 
when zoning GameLib:GetPlayerUnit() is nil for a second or so. it makes this bug out when using recall.
fix:
```Lua
local PlayerUnit = GameLib:GetPlayerUnit()
if PlayerUnit ~= nil then
  local FocusUnit = PlayerUnit:GetAlternateTarget()
  if DestroyedUnit == FocusUnit then
    FocusUnit:SetAlternateTarget(nil)
  end
end
````