WaveformArea/WaveformGroup Drang And Drop fixes

[fredzo]

Hi Andrew,

This PR is a rework of the way drag and drop is handled in WaveformArea and WaveformGroup.
It fixes #642, #636 and #833.
What's been done:

• Added a new "insertion line" visual feedback when dragging an stream in a waveform area to show where to insert the stream in the channel list.
• A stream can now be moved within the same waveform area to change it's position in the channel list.
• Added a visual feedback (not_allowed cursor) when trying to add a stream that is already present in a waveform area.
• Replaced original CenterLeftDropArea() with a zone that covers all the waveform area with the insertion point line feedback.
• Reduced the size of EdgeDropArea to 10% of the area to ease the use of the center drop when the waveform area is small.
• CenterRightDropArea (used for split) is now available at the top of each waveform area and at the bottom of the last, allowing the user to choose at which point the new waveform area has to be created.
• CenterRightDropArea height is dynamically reduced to accommodate waveform area with small heigth
• Edge and Split drop areas have priority over Center drop area.