kcms/componentchooser: unify combobox lengths
Otherwise the comboboxes can have different lengths, making the presentation kind of ragged-looking.
It would get even worse if and when frameworks/qqc2-desktop-style!111 (merged) is merged, as that merge request removed a minimum sizing hack that many comboboxes are currently implicitly relying on.
This MR fixes those issues.
| Status quo | With QQC2 merge request | After |
|---|---|---|
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