Add more validation rules for view construction (thanks to feedback in #519)

[seratch]

Summary

@ruberVulpes
Thank you very much for your great feedback: #519 (comment)

This pull request adds more validation rules in View objects to let developers know errors with more sufficient information. Also, I've updated the views_open API method in WebClient to accept View objects as well.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@61b62fb). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #689   +/-   ##
=========================================
  Coverage          ?   85.40%           
=========================================
  Files             ?       19           
  Lines             ?     2624           
  Branches          ?      249           
=========================================
  Hits              ?     2241           
  Misses            ?      289           
  Partials          ?       94           

Impacted Files	Coverage Δ
slack/web/classes/views.py	85.95% <100.00%> (ø)
slack/web/client.py	94.57% <100.00%> (ø)
slack/web/classes/dialog_elements.py	94.44% <0.00%> (ø)
slack/errors.py	100.00% <0.00%> (ø)
slack/web/slack_response.py	98.00% <0.00%> (ø)
slack/web/classes/dialogs.py	83.33% <0.00%> (ø)
slack/web/classes/attachments.py	98.92% <0.00%> (ø)
slack/rtm/client.py	83.33% <0.00%> (ø)
slack/signature/__init__.py	100.00% <0.00%> (ø)
... and 11 more

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