Refactor how the present_method is selected

[almarklein]

This helps with #138. The logic surrounding the selection of the present_method was not great, with some backends taking the argument in their __init__ to handle it. Now the base class handles it uniformly. Nevertheless, the backend eventually decides what method is selected, and it is clearly defined when this happens, so the backend can make preparations. E.g. if Qt uses 'bitmap', the winid is not even queried.

Very concrete:

• Rename canvas._rc_get_present_methods() -> _rc_present_info(present_methods)
• Previously: the base class queries the canvas (backend) for the methods it supports and then selects a method.
• Now: the base class asks the canvas (backend) to select a method, providing the available methods (based on what the context supports, and user overrides).