[Android] Implemented sf::Window::hasFocus()

[MarioLiebisch]

This is an addition to PR #613 and implements sf::Window::hasFocus() for Android.

[TankOs]

This looks very simple, nice. :) If there are no side effects, i.e. the program is always notified of the focus change by events, then this could work. No Android expert however.

[MarioLiebisch]

Let's state it a bit differently: If that fails, you're very likely without a valid render context, so having that flag return the correct value is least of your concerns. :)

[LaurentGomila]

Well, in fact Window::hasFocus() should always be implementable on top of focus events. If not, it means that the events themselves are not reliable and should be fixed.

[TankOs]

In this case: 👍 This can even be included in 2.2, what do you think?

[MarioLiebisch]

IMO a must-have. The previous "default" behavior of always returning false is quite misleading and might cause "unexpected" behavior for users.

[Bromeon]

This can even be included in 2.2, what do you think?

Yes. Maybe we should do the same for iOS.

[LaurentGomila]

Yes. Maybe we should do the same for iOS.

It's already on my TODO list.

[eXpl0it3r]

This PR has been added to my merge list, meaning it will be merged very soon, unless someone raises any concerns.

[eXpl0it3r]

Merged in ab4d4d7