Map BarModelEditor (aka "fader" even though it's a different thing) progress directly to cursor

[sqrvrt]

output.mp4

List of sins:

• BarModelEditor::setPositionAbsolute feels like it shouldn't exist, or at least should be in FloatModelEditorBase
• BarModelEditor::setPositionAbsolute steals some code from BarModelEditor::paintEvent
• BarModelEditor::setPositionAbsolute steals everything else from FloatModelEditorBase::setPosition
• BarModelEditor::mouseMoveEvent is a very close copy of FloatModelEditorBase::mouseMoveEvent, but position isn't relative
• single click doesn't change the value, only dragging does
• undocumented