Better alternative technique for dragging the keyboard on iOS 9/10

[dzenbot]

Based on #355, this PR includes a more reliable solution for the keyboard panning feature:

• The keyboard stub view is added to the key window's view hierarchy instead. This is particularly helpful for overlapping to any other view, specially when using UITabBar, view controller containment or modal presentations.
• The stubbing logic is now controlled in SLKTextViewController.
• Tested on iOS 7, 8, 9 and 10.

Downsides:

• Because we perform a keyboard view snapshotting, custom keyboard with animations look frozen while dragging the keyboard (e.g. PopKey, GIF Keyboard, etc.).
• Sometimes, the native keyboard's translucency is noticeable lost (depends on the content below the keyboard). This is not a big deal but obviously, not ideal.
• Since we access the keyboard's window, things can break if the UIApplication's windows hierarchy change in future iOS releases. We will be monitoring this and try our best to patch.

Closes #355

[bradya]

We've decided against merging this in for now

[HansBrende]

@bradya @dzenbot What's the status of this behavior for iOS 9/10? Did you guys just give up on this or is something else in the works?

[efremidze]

I've created a branch with interactive keyboard panning support based on what u've proposed.
https://github.com/efremidze/SlackTextViewController/tree/feature/keyboard-panning