Improve mouse keys docs and constant speed mode

[Markus00000]

General changes

• Make default speed settings more consistent across modes:
  • In accelerated and constant+momentary mode, one setting is faster than unmodified speed and two settings are slower
  • In constant+tap-to-select mode, there is no unmodified speed
• Improve documentation

Changes to constant speed modes

• Add an unmodified speed that is used in momentary mode when no KC_ACLx key is pressed
  • This gives users another speed setting without introducing new key codes
  • Previously, one key code (KC_ACL1) had no effect because its speed was identical to the unmodified speed, which could confuse users and made one key code useless
• The speeds are named 0, 1, 2, thus corresponding to their respective key codes KC_ACL0, KC_ACL1, KC_ACL2
  • Easier to configure as one no longer needs to translate between speed descriptions (“SLOW”) and numbers (KC_ACL0) and, for example, a user’s fastest speed can no longer have SLOW in its variable name
  • Furthermore, the SLOW, MEDIUM, FAST naming scheme did not indicate where the unmodified speed in momentary mode should be placed in the series
• Use sensible default settings (YMMV…)

(The original PR has since been merged. Consequently, the following description is no longer useful.)

This recreates pull request #2246 from @JonArintok (unresolved conflicts) with few changes.

Mousekeys can be configured to use three different, constant speeds for cursor movements and wheel scrolling.

(I am not sure if the long if-else blocks in mousekey.c and mousekey.h should be refactored.)

[skullydazed]

The code looks great. If we can fix up the docs a little bit this will be a perfect PR.

[Markus00000]

@skullydazed I have incorporated your suggestions.

I have made numerous changes to the documentation and renamed MK_MOMENTARY_ACCEL to MK_MOMENTARY_SPEED as there is no acceleration involved.

Would it be useful to add the configuration options in a format that allows copy and paste into config.h? Writing 14 definitions is cumbersome.

#define MK_3_SPEED
#define MK_MOMENTARY_SPEED
#define MK_C_OFFSET_SLOW   1
#define MK_C_INTERVAL_SLOW 100
...

Please let me know if you notice any issues or if you have suggestions.