Don't split terminal endlessly.
Don't split terminal endlessly.

Summary: Previously terminology would split the terminal as many times as user hit split button/shortcut. Now splitting is limited to only when there are at least 2 cells left. (horizontally or vertically) Also Fixes T2638

Reviewers: billiob

Maniphest Tasks: T2638

Differential Revision: https://phab.enlightenment.org/D2915

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