Keymap: move builtin keymaps from C to Python

This should be purely an implementation change,
for end users there should be no functional difference.

The entire key configuration is in one file with ~5000 lines of code.
Mostly avoiding code duplication and preserve comments and utility
functions from the C code.
It's a bit long but for searching and editing it's also convenient to
have it all in one file.

Notes:

- Actual keymap is shared by blender / blender_legacy
  and stored in `keymap_data/blender_default.py`

  This only generates JSON-like data to be passed into
  `keyconfig_import_from_data`, allowing other presets to load and
  manipulate the default keymap.

- Each preset defines 'keyconfig_data'
  which can be shared between presets.

- Some of the utility functions for generating keymap items still
  need to be ported over to Python.

- Some keymap items can be made into loops (marked as TODO).

See: D3907

release/scripts/startup/bl_ui/space_userpref.py

@@ -1055,11 +1055,6 @@ class USERPREF_MT_keyconfigs(Menu):
     preset_operator = "wm.keyconfig_activate"
 
     def draw(self, context):
-        props = self.layout.operator("wm.context_set_value", text="Blender (default)")
-        props.data_path = "window_manager.keyconfigs.active"
-        props.value = "context.window_manager.keyconfigs.default"
-
-        # now draw the presets
         Menu.draw_preset(self, context)