Qt: Change the file picker button label to "Browse..."

[lemondrops]

Summary

Change the text on the button that invokes the file picker to the more standard "Browse..."

Checklist

• Closes #xxx
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/

References

N/A