Avoid limiting the size of the Preview Workspace combobox on Elwin

[robertapplin]

Description of work

This PR ensures the Preview Workspace combobox in Elwin has a dynamic size which adjusts according to the size of the GUI. This prevents the name of the selected workspace above the embedded plot from being cut off unnecessarily.

Fixes #38034

To test:

1. Interfaces->Inelastic->Data Processor
2. Go to Elwin tab
3. Check the QComboBox takes up most the space above the Embedded plot

This does not require release notes because it is a regression.

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[cailafinn]

Functionally tested on macOS. Size of combobox is no longer limited and fills the space properly. Code changes look good.