Sample clips lose start/end trim when file is changed

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### Bug Summary

Sample clips do not preserve start/end trim when a sample is loaded into them.

#### Steps to reproduce

- Add a sample clip and load a sample
- Trim slightly from left
- Double click the clip and load another sample (for example, the original sample but slightly edited or at a different file location)

#### Expected behavior

Trim is preserved, or an option to preserve it is presented.

#### Actual behavior

Start and end offset are set to zero, with clip start position remaining unchanged.

#### Affected LMMS versions

1.3 alpha 1, presumably all previous versions.

Reproduce this in LMMS by loading a sample into a sample clip, trimming it, and then loading another file. Trace the sample clip file-loading path and determine where the start and end offsets are reset. Done means the trim is preserved, or the user is given an explicit option to preserve it, with the reported reproduction no longer resetting both offsets to zero.

Written by the indexing model from the issue text.