More de-deprecations

TagLib: switch from deprecated properties->length to properties->lengthInMilliseconds() (introduced in 1.10) Update tests accordingly now that the length is read with greater resolution.

Update gmock tests to use new type MOCK_METHOD definitions (where necessary) to silence hundreds of overload warnings during compile. Requires gmock 1.10, released in 2019.

Add an ifdef to avoid some deprecation warnings on ffmpeg 5.1 and later

Fix some hidden functions, deprecated functions and other miscellanous compile warning cleanup.

Multi-key QHashes made QMultiHashes, or the multikeyness removed; now complete.

Practically only deprecated Qt thing left is QNetworkConfigurationManager, for which's usecases the replacement QNetworkInformation was introduced only in Qt 6.1, so that'll wait.