deprecated AbstractUIPlugin#imageDescriptorFromPlugin

This removes the generated method using the deprecated API from the
templates
Also removes the unnecessary import in the generated activator
Updates the mail RCP example to load the image descriptor with standard
runtime and JFace API (available in all relevant JFace versions)

Potential followup: Remove now unnecessary generateTemplate parameter
from
PluginClassCodeGenerator constructor

Change-Id: Ieee2c65a36c741b9e15bf69ba403c19297a2c345
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
(cherry picked from commit 81b03fe4582cfc367dfe8d292bea5349e5b7552d)