445657: Failed to download image for entry 'Groovy-Eclipse for Juno 2.7.1' (321859) f... (err_grp: a9fe30d6)

- downgrade icon download problem to a warning

Bug: 445657
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=445657
449671: [mpc] Unhandled event loop exception when switching marketplaces

- fire events on selection change
- restore last selection properly on cancel

Bug: 449671
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=449671
447100: [mpc] Unhandled event loop exception

- guard against disposed shell

Bug: 447100
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=447100