Make PyGMT stateful

I'm unsure if "_stateful_" is the appropriate word here. What I really mean is to let PyGMT keep track of its stats and some information, then later PyGMT calls can know what already happened.

I think the "stats" can be useful in many cases. Here are some quick ideas. Let's say we have a dict `pygmt.__stats__` for PyGMT and a dict `fig.__stats__` for each Figure instance, which can store anything we want. The variable names have double underscores so they are for internal use only, but it's also possible to make them public to users.

1. `pygmt.__stats__.call_history` can be a list of commands that are passed to the GMT API via `call_module`. The list can be used to generate the equivalent bash script, although we can't handle virtual files and any other Python data structures.
2. `fig.__stats__.projection`: the projection of the current figure
3. `fig.__stats__.region`: the region parameter of the current figure (c.f. https://www.pygmt.org/dev/api/generated/pygmt.clib.Session.extract_region.html)
4. `fig.__stats__.dimensions`: width and height of the current figure/frame/subplot/inset. Users can access these values via public functions, and then can do some mathematic calculates (e.g., to have an inset map with a width of 0.25 * *w* or shift the plot origin by 1.2 * *w* (*w* is the width of current figure stored in `fig.__stats__.dimensions`)).

I believe there are more interesting ideas about a stateful PyGMT, so thoughts and comments are welcomed.