Vertical panels are special: While horizontal panels are generally
almost always only used in configurations ranging from one to three
text lines in height, vertical panels are used in a wider range
from very narrow to very wide, depending on whether users want to
fit only icons or also text (which needs ample horizontal space to
be useful).

In a wide vertical panel, it's inappropriate for launcher icons to
grow beyond a certan size, as they otherwise occupy too much
vertical space. For that reason D4550 introduced an icon size setting
in the Task Manager applet, shown only when the applet is embedded
into a vertical panel.

This patch brings the same setting to Kicker and App Dash, with the
same smallMedium default. A narrow vertical panel which doesn't fit
text labels or uses Icons-only Task Manager will therefore get same-
sized icons in those launchers and in the TM. A wider vertical panel
won't see the launcher icon grow beyond the TM's icon size if both
are left as default, with settings allowing users to manage the two
individually.

CCBUG:378443

A similar change will need to be done for Kickoff, which uses the
default CompactRepr instead.