Replace use of depends_on in Traits Properties with observe

[aaronayres35]

This PR replaces almost all uses of depends_on with observe, aside from this:

chaco/chaco/base_1d_plot.py

Line 40 in 99109d9

index_range = Property(depends_on="index_mapper.range")

changing this line resulted in he following errors:

Details

======================================================================
FAIL: test_linescatter_1d_set_index_mapper_notifies_index_range (chaco.tests.test_line_scatterplot.LineScatterPlot1DTest)
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Traceback (most recent call last):
  File "/Users/aayres/Desktop/chaco/chaco/tests/test_line_scatterplot.py", line 185, in test_linescatter_1d_set_index_mapper_notifies_index_range
    self.linescatterplot.index_mapper = LinearMapper(range=new_range)
  File "/Users/aayres/Desktop/traits/traits/testing/unittest_tools.py", line 118, in __exit__
    raise self.failureException(msg.format(*items))
AssertionError: Change event for index_range was fired 2 times instead of 1

======================================================================
FAIL: test_scatter_1d_set_index_mapper_notifies_index_range (chaco.tests.test_scatterplot_1d.Scatterplot1DTest)
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Traceback (most recent call last):
  File "/Users/aayres/Desktop/chaco/chaco/tests/test_scatterplot_1d.py", line 181, in test_scatter_1d_set_index_mapper_notifies_index_range
    self.scatterplot.index_mapper = LinearMapper(range=new_range)
  File "/Users/aayres/Desktop/traits/traits/testing/unittest_tools.py", line 118, in __exit__
    raise self.failureException(msg.format(*items))
AssertionError: Change event for index_range was fired 2 times instead of 1

======================================================================
FAIL: test_text_1d_set_index_mapper_notifies_index_range (chaco.tests.test_text_plot_1d.TextPlot1DTest)
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Traceback (most recent call last):
  File "/Users/aayres/Desktop/chaco/chaco/tests/test_text_plot_1d.py", line 133, in test_text_1d_set_index_mapper_notifies_index_range
    self.textplot.index_mapper = LinearMapper(range=new_range)
  File "/Users/aayres/Desktop/traits/traits/testing/unittest_tools.py", line 118, in __exit__
    raise self.failureException(msg.format(*items))
AssertionError: Change event for index_range was fired 2 times instead of 1

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Note that the trait is changed twice with observe. assertTraitChanges conveniently provides the events that were triggered. Printing these for one of the tests, we see:

[
(<chaco.line_scatterplot_1d.LineScatterPlot1D object at 0x7fcfc7e7c258>, 'index_range', <undefined>, <chaco.data_range_1d.DataRange1D object at 0x7fcfc7e7c308>),
(<chaco.line_scatterplot_1d.LineScatterPlot1D object at 0x7fcfc7e7c258>, 'index_range', <undefined>, <chaco.data_range_1d.DataRange1D object at 0x7fcfc7e7c308>)
]

ie the exact same event is occurring twice.

I spent a bit of time trying to understand this, but still am not sure where the problem is coming from.
For now, I figure we can go ahead with the rest of the changes and save this for another day.

Additionally, this PR also changes a property to observe selected_mask not selection_mask as no selection_mask is defined on the class but selected_mask is (I assume this was the original intention, +1 for observe catching a potential silent bug).

[aaronayres35]

One thing I am now considering is that it is possible other changes are having a similar effect (triggering a trait to be changed twice), but may just not be explicitly tested. I do not know if this behavior is really problematic or not

[rahulporuri]

LGTM