Center title for px.imshow

[s2t2]

Possible fix for #4303

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[empet]

It's not only px.imshow that aligns the plot title at left. The left aligned title is set by default in all plotly templates:
https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/templategen/definitions.py#L120
https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/templategen/definitions.py#L399
https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/templategen/definitions.py#L470
https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/templategen/definitions.py#L551

Starting with plotly.py, version 4.0, when templates were introduced, I set every plot title at 0.5, to get it centered. Only section titles in newpapers, books, journal papers are left aligned. In my opinion the left aligned title was an uninspired choice.
The most convenient would be to set in each plotly template definition, title_x=0.5, and users who want a non-standard alignment to set it manually.

[s2t2]

Sounds good @empet if there's anything you would like me to help with (like providing guidance around testing and passing the build), please let me know.