desktoppackage: Support the "osdAdditionalText" property in OsdItem

This is currently only used by the mediaPlayerVolumeChanged method, where playerName is relayed to the OSD as additional text. We can make use of this also for an upcoming screenBrightnessChanged method addition.

In the QML, the label item is pulled upwards in the code so it can appear on top of the progress bar, instead of being mutually exclusive. Preferred height is adjusted to also take implicitHeight into account, rather than only the size of the icon.

This commit mirrors the corresponding plasma-workspace commit to an extremely similar OsdItem in its lookandfeel subdirectory.