Refresh more deprecated codepaths.

These modifications remove usage of many, but not all, remaining deprecated Qt5 functions as of 5.15, and also usage of some deprecated KF5 and TagLib functions. The changes should be generally pretty straightforward, and everything compiles and works-for-me-so-far - however, there are some things (e.g. iPod-functionalities) that I won't even be able to actually test.