Code refactoring

[AnErrupTion]

Resolves: None (mostly spontaneous)

This is a relatively "minor" PR (in changes), but it's a rather big refactoring. Essentially, I've ran clang-tidy over the code base and fixed what I thought would be good to fix. Notably, I think this could help for #56. C++20 is probably more restrictive towards the syntax and stuff like this.

Alongside this, I've made another minor change, which is to let the startup splash screen show until the main project window is shown. In my opinion, this makes more sense, so that if the splash screen closes unexpectedly, you immediately know there's a problem, for example.

Finally, I've updated the BUILDING.md file to add a note, which says that building wxWidgets as a static library will cause Saucedacity to fail to launch.

• I made sure the code compiles on my machine*
• I made sure that my contributes are licensed under GPL v2 or later.*
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"

*Indicates required.