Increase SLKTextView's influence over calculating its height.

[lukkas]

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PR Summary

Change doesn't affect the functionality in any way. It moves SLKTextView height calculation logic to SLKTextView class itself. This adds flexibility when subclassing SLKTextView.
Example:
When adding custom line spacing using NSAttributedString and NSParagraphStyle to text in text view, there is currently no way to change its height in order to accommodate text formatted that way.

[dzenbot]

LGTM but please update the recent conflict.