Krasimir

#24908: Validate saved files and alert user

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Resolves: #24908 Initial commit to demonstrate the idea and start refining it. - [X] I signed the [CLA](https://musescore.org/en/cla) - [X] The title of the PR describes the problem it addresses...

Undocked panels should remember their position/size

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Resolves partially: #11242 & #26678 (partially because docked panels are still a problem) Resolves: #18345 (however the height is not retained because the Mixer auto-calculates it) Resolves: #27570 --- NOTE:...