Add support for text styles on Win32+Celiagg

[jwiggins]

Fixes #643

Celiagg 2.1.0 introduces a new font object for win32 which allows specification of bold/italic styles.

[jwiggins]

This will have to wait for the celiagg release, or else I'll have to add a check for the presence of FontWeight:

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ERROR: test_text_clip (kiva.tests.test_celiagg_drawing.TestCeliaggDrawing)
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Traceback (most recent call last):
  File "D:\a\enable\enable\.edm\envs\enable-test-3.6-pyqt\lib\site-packages\kiva\tests\drawing_tester.py", line 127, in test_text_clip
    self.gc.set_font(font)
  File "D:\a\enable\enable\.edm\envs\enable-test-3.6-pyqt\lib\site-packages\kiva\celiagg.py", line 624, in set_font
    weight = agg.FontWeight.Regular
AttributeError: module 'celiagg' has no attribute 'FontWeight'

[rahulporuri]

LGTM. I tested this locally on my windows machine and it worked like a charm - not with celiagg 2.0 on edm but 2.1.0 on pypi. Tested using the kiva_explorer with the default "Times New Roman" font.

[jwiggins]

Thanks for the testing/review @rahulporuri!