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 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".