new gamepack-manager script to download and install game packs

new gamepack-manager script to download and install game packs

New gamepack-manager script to download and install game packs
--------------------------------------------------------------

Just run `./gamepack-manager -h` to get an extensive help.

It obsoletes:

- `download-gamepacks.sh`
- `install-gamepack.sh`
- `install-gamepacks.sh`

New cmake options to configure game pack downloading
----------------------------------------------------

- `GAMEPACK_LICENSE`
- `GAMEPACK_NAME`

They must be used with `DOWNLOAD_GAMEPACKS=ON`

If set to `none`, those extra filters are not used.
By default `GAMEPACK_LICENSE` filters for free licenses only.
By default `GAMEPACK_NAME` does not filter anything.

Examples
--------

Configure to only fetch free game packs:

```sh
cmake .. \
	-DDOWNLOAD_GAMEPACKS=ON \
	-DGAMEPACKS_LICENSE=free \
	-DGAMEPACKS_NAME=none
```

Configure to only fetch Xonotic and Unvanquished game packs:

```sh
cmake .. \
	-DDOWNLOAD_GAMEPACKS=ON \
	-DGAMEPACKS_LICENSE=none \
	-DGAMEPACKS_NAME="Xonotic Unvanquished"
```

Configure to only fetch GPL game packs and Quake2 game pack:

```sh
cmake .. \
	-DDOWNLOAD_GAMEPACKS=ON \
	-DGAMEPACKS_LICENSE=GPL \
	-DGAMEPACKS_NAME="Quake2"
```

Fetch the game packs:

```
make game_packs
```

Note that it also works on `install` stage.