Adds various clock options.

[matham]

Please read the docs at of clock.py (https://github.com/matham/kivy/blob/4e893a4f40d279d58a376ca4f2fe88b45e2ded37/kivy/clock.py#L221) added by this PR to understand what it does.
A quick summary from there is:

The kivy clock type to use can be set with the kivy_clock option the
:mod:~kivy.config. If KIVY_CLOCK is present in the environment it
overwrites the config selection. Its possible values are as follows:

• When kivy_clock is default, the normal clock, :class:ClockBase,
  which limits callbacks to the maxfps quantization - is used.
• When kivy_clock is interrupt, a interruptible clock,
  :class:ClockBaseInterrupt, which doesn't limit any callbacks to the
  maxfps - is used. Callbacks will be executed at any time.
• When kivy_clock is free_all, a interruptible clock,
  :class:ClockBaseFreeInterruptAll, which doesn't limit any callbacks to the
  maxfps in the presence of free events, but in their absence it limits events
  to the fps quantization interval - is used.
• When kivy_clock is free_only, a interruptible clock,
  :class:ClockBaseFreeInterruptAll, which treats free and normal events
  independently; normal events are fps limited while free events are not - is
  used.

Also fixes #4417.