fix: UTF-8 sources in MSVC

[csett86]

This forces MSVC to use UTF-8 as source encoding, and with that the degree symbol finally works also when compiled with msvc.

Before:

After:

Closes: #315

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[what-the-diff]

PR Summary 🌟

• UTF-8 Support for MSVC Builds 😃
  We've added a line to common.pri, ensuring consistent UTF-8 encoding for special symbols and characters (like °) in source files on MSVC builds. No worries, g++ users - this change doesn't affect you! We found this helpful change from a [Stack Overflow post](https://stackoverflow.com/questions/48705747/how-utf-8-may not work in qt 5). 🎉

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[DSCaskey]

I tested this and it also takes care of the base label strings - so it looks good to me to merge.