…lipboard functions

+ demo window example for the same

This functionality should work at least on Windows and macOS out of the
box as ImGui sets up default clipboard functions at init time.  Support
for overriding the clipboard callbacks is not implemented (although
still on the table in future versions.)

for #16

DrawList methods:

  path_clear
  path_line_to
  path_line_to_merge_duplicate
  path_fill_convex
  path_fill_concave
  path_stroke
  path_arc_to
  path_arc_to_fast
  path_elliptical_arc_to
  path_bezier_cubic_curve_to
  path_bezier_quadratic_curve_to
  path_rect
  push_clip_rect
  push_clip_rect_full_screen
  pop_clip_rect
  push_texture
  pop_texture
  get_clip_rect_min
  get_clip_rect_max
  add_draw_cmd

Also add a Python port of the ImGui custom rendering example Primitives tab.

for #16

Also adds demo window support (see layout.py)

for #16

- remove macos-related skips
- remove useless pp selectors
- upgrade cibuildwheel version

  IO.add_focus_event
  IO.add_input_characters_utf8
  IO.add_key_analog_event
  IO.add_mouse_source_event
  IO.clear_events_queue
  IO.clear_input_keys
  IO.clear_input_mouse
  IO.set_app_accepting_events