PlainMessageDialog should act as a replacement of MessageDialog.
Works with a builder pattern to create an immutable dialog.

Two usages of the new dialog in:
- File Association Preference Page: when creating an already existing
file association
- Perspective Preference Page: when deleting a perspective

Change-Id: Idb9f78b4e98b945000d19ed6384465099ddb98aa
Signed-off-by: Marcus Hoepfner <marcus.hoepfner@sap.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/170943
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Lars Vogel <Lars.Vogel@vogella.com>