Reset waveform cache after changes in a clip

[vsverchinsky]

Resolves: #6373

• [x] I signed CLA
• [x] The title of the pull request describes an issue it addresses
• [x] If changes are extensive, then there is a sequence of easily reviewable commits
• [x] Each commit's message describes its purpose and effects
• [x] There are no behavior changes unnecessary for the stated purpose of the PR

Recommended:

• [x] Each commit compiles and runs on my machine without known undesirable changes of behavior