ui/qt6: replace deprecated count() with size()

[Sunderland93]

Description

count() is deprecated since Qt 6.4. Use size() instead

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[LibretroAdmin]

Do these functions have any functional differences to strlen? What if we just used that instead so the code could work for all Qt versions? We would of course have to take that size() use out of the for loop declaration and put it in a variable outside instead

[Sunderland93]

size()/count() in Qt return the container’s length in O(1) and work with QString/QByteArray, while strlen() only works with null-terminated char* and is O(n). For cross-Qt compatibility, keep size() (cache it if needed for loops).

[LibretroAdmin]

Alright, size() seems to go as far back as Qt 4.8 so I think we are good then in terms of backwards compatibility (correct me if I'm wrong)