Added the ability to request focus lock for keyboard events

Keyboard event handlers have a new system member that stores a pointer to the KeyboardInputSystem it is registered to.
By using this pointer handlers can request and release a focus lock.
In addition there is a new callback that is used when keyboard focus has been lost via a lock or window focus loss.

Also WebGUIView now checks for focus for keyboard events as well as the character events it did before.
Implemented Mouse focus lock

Works just like keyboard focus lock.

Also had to rename the respective system pointers to avoid ambiguity.
The are now mouseSystem and keyboardSystem respectively.
OS notifies KeyboardEventSystem on a window focus loss
Merge branch 'develop' of https://github.com/adam4813/Sigma into vm