[3.0] qt: use type-safe connections in most places

Most signal-slot connects are rewritten from the old string-based runtime selection to the compile-time selection introduced in Qt 5. This is in spirit effectively a backport of [1] and [2], but written from scratch due to excessive conflicts due to code divergence. This will detect some of the issues mentioned in [3] (at least the "Open Recent Media" doing nothing), due to detecting the removal of the slots at compile time (and thus failing to compile, instead of producing a binary that doesn't work).

[1] eb0b785f [2] 77b75273 [3] !7754 (comment 499875)