Add xcb dynamic loading support

[mwestphal]

I agree that my contributions are licensed under the MIT License.
You can find a copy of this license at https://opensource.org/licenses/MIT

This adds supports for loading xcb dynamically instead of linking against it, by providing all needed symbols as function pointers.

TODO: Error management

Fix: #98
Fix: f3d-app/f3d#3326

[mwestphal]

Error management is not done, I did not find a way to convey an clean error if the lib or a symbol is not available. There is an error code stuff but I'd need to be able to abort right during the manager creation.
In that case I usually would use an exception and document it in the API doc, but there is not a single exception in your API, please let me know.

@dacap

[mwestphal]

@Meakk please take a look

@dacap please review

[Meakk]

a few comments

[Meakk]

Looks good to me but I think @dacap wanted to have a CMake variable to switch between link and dynamic load mode.
Let's see what they think about dropping the link mode.

[mwestphal]

Looks good to me but I think @dacap wanted to have a CMake variable to switch between link and dynamic load mode. Let's see what they think about dropping the link mode.

Right, completely forgot about that.

[mwestphal]

@dacap in the meantime I have added a simple exception system