Buttons jump around when skipping songs

[elementaryBot]

The sound notification indicator should ellipsize song names and always have a fixed witdth to avoid having the buttons jumping around when skipping songs.

Ellipsize is already implemented, but it isn't strict enough to keep the buttons from jumping around.

Launchpad Details: #LP1578440 Zisu Andrei - 2016-05-05 00:01:21 +0000

[bluesabre]

Since ellipsizing is based on the number of characters, maybe this could be better handled with a ScrolledWindow and faded overshoot on the right?

For the fade idea:
https://blog.elementary.io/images/introducing-elementary-os-5-1-hera/popover-fade.png

Here's a screenshot of the ScrolledWindow implementation, but without the fade: