Timeline: restore Item#message height shrink ability

[rpallai]

This got broken by dbda393 which introduced circular dependency on height: Item#message height was dependent on childrenRect.height but it included a MouseArea which was dependent on parent.height.

It turned out by 87b18d9 via changing discardButton type from ActiveLabel to ToolButton which is taller and forces sent messages to shrink when it gets invisible (in normal circumstances, shortly after it appears).

[KitsuneRal]

Now that I look once again at the code, I start wondering if we really need that many TimelineMouseAreas.

1. Does it make sense to consolidate similar mouse areas into one under the top-level Item with anchors.fill: fullMessage instead of having one mouse area per each piece of the message? (and then this PR would be unnecessary in its current form)
2. We now have 3 TimelineMouseAreas under textField - I strongly suspect we can consolidate them into one.

[rpallai]

1. Does it make sense to consolidate similar mouse areas into one under the top-level Item with anchors.fill: fullMessage instead of having one mouse area per each piece of the message? (and then this PR would be unnecessary in its current form)

You are absolutely right. The reason of having different MouseAreas was historical but the code was reworked several times since and it isn't necessary anymore. I forgot to consolidate them. Thanks!

2. We now have 3 TimelineMouseAreas under textField - I strongly suspect we can consolidate them into one.

I agree, it's a practical simplification. Pushed.