fix: unify label widths when widgets passed to Container constructor

[tlambert03]

Container(widgets=[...]) rendered with different label/control alignment
than an empty Container populated with append(). During init, widgets
are inserted before _initialized is set to True, but _unify_label_widths()
is a no-op until _initialized is True, so label widths were never unified
for constructor-supplied widgets.

Call _unify_label_widths() once after initialization completes.

Fixes #729

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.27%. Comparing base (2d985e7) to head (b0b9943).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #730   +/-   ##
=======================================
  Coverage   89.27%   89.27%           
=======================================
  Files          40       40           
  Lines        4821     4822    +1     
=======================================
+ Hits         4304     4305    +1     
  Misses        517      517           

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