Fix for User Function Dialog button hidden behind scroll bar

[SilkeSchomann]

Description of work

On macOS, a scroll bar made it impossible to click the User Function Dialog button in the Formula box for the fit toolbar on a plot. Removing the 0 spacing setting and increasing the margin to the right for the User Function Dialog button solved this problem.

Fixes #37579

To test:

Follow the instructions in the original issue on macOS.

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