[applets/batterymonitor] PowerProfileItem: Elide slider labels

There are many languages (including Russian) where three three power
profile titles do take much more space than it is available in applet.
RowLayout's spacing ensures they do not stick to each other side by
side.

Ideally, we should have "shrinking labels" which are capable of
adjusting their text's kerning based on available space. In the
meantime, localization teams are advised to use as short profile names
as possible without loosing their meaning.