PR: Reapply dock tabbar style if previous session was a Spyder 6 one (Layout)

[ccordoba12]

Description of Changes

• This is necessary if users start a Spyder 5 session after running a Spyder 6 one before.
• The reason is that we had to bump the window state version in Spyder 6 due to the migration of the Editor to the new API. And that requires reapplying the dock tabbar style because it's reset by Qt when a different state version is used.
• The changes in master to account for the inverse situation (i.e. starting Spyder 6 after a Spyder 5 session) are different. I'll apply them when merging this PR with that branch.

Affirmation

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I certify the above statement is true and correct: @ccordoba12