Cypress Tests for -use_frontend_client_polling variant of visdom

[da-h]

Description

Ensuring functionality also using the -use_frontend_client_polling flag.

Motivation and Context

Regarding the recent proxy-users and an upcoming code simplification of the server socket handlers, I suggest to add some testing of the polling-feature.
I have explicitly only added the "functional tests" to be tested against the -use_frontend_client_polling flag, which I though should be sufficient.

In addition, a few minor changes were necessary in the following test cases:

• the ImagePane tests image_callback and image_callback2 failed now and then for the poller-implementation, but not for the websocket implementation. This was due to two windows being loaded in different orders in both implementation. (The image-panes load slower when using long polling). This PR solves this by adapting all cypress selectors for image panes to use a "pane with an Image" instead of the "first" pane in the list of panes.
• The test for the view modal did not work due to a similar problem: the image pane loaded slower for the poller-implementation. For sake of consistency, I've added wait(500) commands in between the respective pane-creation commands.
• The text_callback test failed randomly as well. This is probably due to too a missing order-enforcement of the events when passing incoming events to the callback. (Not sure here. Is this correct?). This PR increases the delay in-between callback keystrokes to 200ms (from previously 50ms) to solve the issue of inconsistent tests.

How Has This Been Tested?

Testing using this very PR-action, which should already include the test.
Also, to test for consistency, I've triggered the exact same action onto 10 branches, see for instance here

Types of changes

• Test cases
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactor or cleanup (changes to existing code for improved readability or performance)

Checklist:

• I adapted the version number under py/visdom/VERSION according to Semantic Versioning
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[JackUrb]

Happy to have more tests here