Translucent TCOs for visible overlap

### Enhancement Summary
In the Song Editor, it is possible to move one segment or TCO over another so that they overlap. In this case, one of the TCOs shows on top, and the other is hidden behind it. Most other DAWs use some sort of transparency/translucence so as to show both patterns, but LMMS hides some or all of one of the patterns altogether.
When the piano roll was being modified, [this](https://github.com/LMMS/lmms/issues/2777#issuecomment-221972615) comment mentioned how overlapping notes don't appear to overlap, and thus notes were made translucent based on their volume. I'm suggesting something similar for the clips in Song Editor.

#### Justification
When two TCO's overlap, then the only thing we have as visual indication is this slight shadow, and an incomplete selection box:
This only works if one of the TCO's is longer than the other, and the longer one is at the back; and because the shadow is just barely visible, the selection box determining only works when the longer TCO is selected.
Other than this, there is no visual indication that there is a hidden TCO behind another.
(This is also assuming both TCO's have the same start point. This will also apply if the end of one of the TCOs goes over the start of another, because there's no indication where they start or end.)

#### Screenshots
Other DAWs' behaviour:

**Reaper:**
Individual:

Partially overlapping:

Fully overlapping (same size):

Fully overlapping (different size):

**Zynewave Podium**
Individual:

Partially overlapping:

Fully overlapping (different size):

(Any other DAWs I have either don't support overlapping, or don't visually represent overlapping well. I don't have any trial versions of any other commercial DAWs and therefore can't provide screenshots of those.)

Start by locating the Song Editor's TCO rendering and review the linked piano-roll discussion about translucent overlapping notes. Done means overlapping clips visibly reveal both patterns, including partial and complete overlap, without relying only on shadows or selection boxes.

Written by the indexing model from the issue text.