Refactor text properties

[hanjinliu]

Closes #39
Also improve performance of vispy text layer.

[codecov]

Codecov Report

Attention: Patch coverage is 90.68627% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 83.87%. Comparing base (1bc0a45) to head (c331e05).

Files	Patch %	Lines
whitecanvas/backend/vispy/text.py	75.40%	8 Missing and 7 partials ⚠️
whitecanvas/layers/_mixin.py	84.61%	1 Missing and 1 partial ⚠️
whitecanvas/backend/bokeh/text.py	98.55%	1 Missing ⚠️
whitecanvas/layers/group/labeled.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   83.88%   83.87%   -0.02%     
==========================================
  Files         162      162              
  Lines       18366    18304      -62     
  Branches     3127     3093      -34     
==========================================
- Hits        15407    15352      -55     
  Misses       2141     2141              
+ Partials      818      811       -7     

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