Split Terminals - allow to select terminal like --select-tab in CLI

This is related to issue #1645

`--split-vertical` and `--split-horizontal` are really nice ! But it feels to me that a select terminal feature is missing.

Typically, if you want to make a 2x2 grid, you can't right now because the first terminal cannot be selected. Hence you can only split it vertically or horizontally, but not both.

Proposal:
```bash
guake --select-tab N --select-terminal M
```
Where M would be an auto-incremented value, very much like the tabs work.

I don't know how complicated it would be to implement, but it would make complex startup-scripts more powerful.

Example for a 2x2 grid :

```bash
# Main terminal has index 0
guake --split-horizontally # Right terminal has index 1
guake --split-vertically # Bottom-Right terminal has index 2, Top-Right keeps index 1
guake --select-terminal 0 # Back to left terminal
guake --split-vertically # Bottom-Left terminal has index 3, Top-Left keeps index 0
```

We end up with this layout:
```
|---|---|
| 0 | 1 |
|---|---|
| 3 | 2 |
|---|---|
```