rosegarden: 17.04 -> 17.12.1

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/6q311ykga5a001jk8x80whj4cr3jp1y5-rosegarden-17.12.1/bin/rosegarden --version` and found version 17.12.1
- found 17.12.1 with grep in /nix/store/6q311ykga5a001jk8x80whj4cr3jp1y5-rosegarden-17.12.1
- found 17.12.1 in filename of file in /nix/store/6q311ykga5a001jk8x80whj4cr3jp1y5-rosegarden-17.12.1

cc "@lebastr"