matroskareadcommon: merge tags in track targets

If a file contains multiple tag blocks with the same track ID, the new tags should be appended to the list of previous tags instead of replacing them.

For example the following structure:

Tags:
├── Tag:
│   ├── Targets:
│   │   └── Track UID: 1
│   └── Simple:
│       └── Tag1: 1
└── Tag:
    ├── Targets:
    │   └── Track UID: 1
    └── Simple:
        └── Tag2: 2

Should result in the extended tags "Tag1=1" and "Tag2=2", but instead the list of extended tags only contains "Tag2=2".