Use the properties in <dependencyManagement> and <pluginManagement>

This allows us to omit the explicit versions of dependencies (but sadly,
not the groupIds) in POMs inheriting from pom-scijava.

Example:

	...
	<dependencies>
		<!-- The version is defined in pom-scijava -->
		<dependency>
			<groupId>${imagej.groupId}</groupId>
			<artifactId>ij-core</artifactId>
		</dependency>
	</dependencies>
	...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>