Bug 577564 - add priority attribute to variableValueEditor

New "priority" attribute is optional integer number 0 to 10 describing
priority of this extension, with 0 being the lowest priority. If
multiple contributions have the same priority, an arbitrary contribution
among them will be selected.

Change-Id: Ib787799214f31423adb66ec1c5bd56f555f97dde
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.debug/+/188423
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>