Update api module docstring for chaco.api

[aaronayres35]

Follow up to #649

This PR populates and move the colormap section to the bottom of the module (although this move may have been unnecessary as I had failed to notice the all previously)
It also moves BandedMapper into the mappers section and ColormappedSelectionOverlay into the visual components section

[rahulporuri]

LGTM with a small comment.

[rahulporuri]

Still LGTM

[aaronayres35]

This test failure has returned...

======================================================================
ERROR: test_dont_crash_on_click (chaco.tests.test_plot.TestEmptyPlot)
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Traceback (most recent call last):
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\chaco\tests\test_plot.py", line 134, in test_dont_crash_on_click
    [(1, 1), (2, 2), (3, 3), (4, 4)],
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\contextlib.py", line 88, in __exit__
    next(self.gen)
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\traitsui\testing\tester\ui_tester.py", line 106, in create_ui
    process_cascade_events()
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\contextlib.py", line 88, in __exit__
    next(self.gen)
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\traitsui\testing\_exception_handling.py", line 74, in reraise_exceptions
    raise RuntimeError(msg)
RuntimeError: Uncaught exceptions found.
=== Exception (type: <class 'AttributeError'>, value: '_QtWindow' object has no attribute 'handler') ===
Traceback (most recent call last):
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\enable\qt4\base_window.py", line 258, in leaveEvent
    self.handler.leaveEvent(event)
AttributeError: '_QtWindow' object has no attribute 'handler'

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I had thought it was consistently occurring, so when it didn't happen on #663 (and on any commits since then...) I had assumed that fixed it.

Looks like this is really an intermittent failure and will require more investigation.

I am tempted to just skip the test on windows/pyqt for now until it can truly be resolved so it doesn't interfere with other PRs. Will do this in a separate PR.