style

Under Ubuntu/Linux having the inset makes it impossible to
hover over the tooltip. Removing this insert enables this under Linux.
EGit had to implement a workaround for this via Bug 516969.

Change-Id: Ibea407bbb047d05da87999a76d8024b7b1dec329
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>