Allow specifying the number of equal parts to split the terminal tab into

As an user, I would like to be able to split the terminal tab into more than two equal parts, e.g. split the terminal vertically into three parts with equal width. Currently Guake only allows splitting in half.

To do this, I propose adding an optional parameter to the `--split-vertical` and `--split-horizontal` command line options to specify the number `N` of equal parts to split into, e.g.

```
--split-vertical [N] Split the selected tab vertically into N equal parts (default: 2).
--split-horizontal [N] Split the selected tab horizontally into N equal parts (default: 2).
```