Widget tooltips

[corranwebster]

With #1013 merged, it's now possible to move tooltip code from Fields to Widgets, so all widgets can have a tooltip. This is PR does a fairly straightforward shift of the code between the two classes, and updates tests and documentation appropriately.

Code that uses tooltips on Fields should notice no change.

[rahulporuri]

@corranwebster CI seems to be failing with wx -

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FAIL: test_field_tooltip (pyface.tests.test_widget.TestWidgetCommon)
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Traceback (most recent call last):
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/testing/widget_mixin.py", line 57, in test_field_tooltip
    self._create_widget_control()
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/testing/widget_mixin.py", line 41, in _create_widget_control
    self.widget._create()
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/i_widget.py", line 134, in _create
    self.control = self._create_control(self.parent)
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/tests/test_widget.py", line 36, in _create_control
    control = wx.Window(parent)
wx._core.wxAssertionError: C++ assertion "Assert failure" failed at /home/vagrant/wxPython-4.0.7.post2/ext/wxWidgets/src/gtk/window.cpp(2474) in Create(): wxWindowGTK creation failed

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[corranwebster]

Yep, pushed a fix (was being lazy about testing locally because I didn't want to clobber my development environment).

[rahulporuri]

Mostly LGTM with a couple of necessary changes

[corranwebster]

Looks like _remove_event_listeners is being called twice.

[corranwebster]

The calling chain for destroy was not correct. The latest commit adds a mixin destroy method that ensures _remove_event_listeners gets called exactly once.

This exposed a bug in the close method of SingleChoiceDialog so this PR also fixes that (the thing being tested wasn't particularly important - it was testing the value of the choice when the user cancels, which we shouldn't really care about at all).

[corranwebster]

Looks like some other double-disconnects on the wx side 😞

[rahulporuri]

LGTM. Thanks for the fixes.