Add a new config option for the icon size in the Places Panel

To change the icon size in the Places Panel to, e.g., 60 pixels, add
the following lines to $KDEHOME/share/config/dolphinrc :

[PlacesPanel]
IconSize=60

A value of -1 means that the standard "Small" size should be used
(which has been the default before this commit).

It is planned to add an "Icon Size" submenu to the context menu of the
Places Panel in KDE 4.10, but we cannot do this in the 4.9 branch
because of the string freeze.

FIXED-IN: 4.9.2
BUG: 301959