JBIDE-17514 JBIDE-17425: Better handling of scroll

This fixes issue of scrollbar not being updated when
content of the scrolled composite is resized.
JBIDE-17474: Bigger size for install/uninstall buttons
JBIDE-17531 : check for disposed widget

Signed-off-by: Fred Bricon <fbricon@gmail.com>
JBIDE-17534: Refresh selection when enabling Early Access