sampleeditor: improved paste (mix, mix overflow, spread)

[coderofsalvation]

Issue: the default 'Paste > Mix' spreads the selection over the destination sample.

This is fine, but a bit unexpected compared the normal wave-editor 'mix paste'

Solution: 'Paste > Mix' has been renamed to 'Paste > Mix spread' and introduced are:

• Paste > Mix (mix without spreading/stretching)
• Paste > Mix overflow (if the clipboard duration is longer, then it folds back into the beginning pacman-style)