Use Git subtree instead of submodule

These commands were run:

```
git submodule deinit bogo-python
git rm bogo-python
git subtree add -P bogo-python --squash https://github.com/BoGoEngine/bogo-python.git master
```

So now basically the bogo-python sub-project is committed directly into ibus-bogo-python. To update it:

```
git subtree pull --prefix=bogo-python --squash https://github.com/BoGoEngine/bogo-python.git master
```