To swap 'split' labels

### To swap 'split' labels
This will make it more intuitive for users.
 - **Split Horizontally** should add **horizontal** divider (split). *This makes this two vertical GTK controls.*
 - **Split Vertically** should add **vertical** divider (split). *This makes this two horizontal GTK controls.*


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Here are examples from other terminal emulators:
 - Terminator:
  ![terminator_split](https://cloud.githubusercontent.com/assets/7157049/3558908/848dd848-0942-11e4-9256-9986e99e5d25.png)

 - Terminology:
  ![terminology_split](https://cloud.githubusercontent.com/assets/7157049/3558909/848df2e2-0942-11e4-88b9-1bf2efa03fe9.png)