applet-frame: don't use GtkBox
applet-frame: don't use GtkBox

WnckTasklist requests minimum width to fit buttons in calculated
columns. If buttons are placed in one row then requested width
will be n buttons * button width.

The problem is that GtkBox will allocate minimum required width
for task list widget. This problem can be seen with default panel
layout - opening enough windows task list buttons will be under
workspace switcher and will not be clickable.

This commit fixes regression introduced in fc8947f392b1.

GNOME/gnome-panel,GNOME/gnome-panel