Feature Request: Break up `FacetX$draw_panels()` into smaller functions

[teunbrand]

The draw_panels() methods are quite beefy with various tasks. Extending facets with custom draw_panels() methods is a bit of a pain due to having to repeat/copy a lot of code. As I understand the code, there are three major steps that should be mostly independent of one another:

• Initialise the panels as a gtable.
• Attach axes to panels.
• Attach strips to panels.

There are more granular steps within each task, but I think the above tasks are good chunks to isolate into separate methods to make facets more extensible. My proposal is to keep draw_panels() for backward compatibility, but add 3 methods for the tasks above that are called in draw_panels().