Refactor normalization of (block) text objects

[TheBB]

Originally reported by: Frank Fischer (Bitbucket: lyro, GitHub: lyro)

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The selection of block text objects is normalized similarly to usual motions. The current ad-hoc implementation should be refactored and properly documented.

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• Bitbucket: https://bitbucket.org/lyro/evil/issue/646

[TheBB]

Original comment by Frank Fischer (Bitbucket: lyro, GitHub: lyro):

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see #643 and #645