Fixed issues in 0.7.2

Issues with corresponding Bugzilla entry

~~bug 419439~~ [Viewers] edges disappear if the source or target item is changed
~~bug 419985~~ GMF dependency in viewers.runtime.zest
~~bug 420934~~ Engine fails during variable unification for convoluted pattern
~~bug 420935~~ Pattern validator trips on convoluted pattern with many == constraints
~~bug 417303~~ Query specification registry should give information on which extension point failed to load
~~bug 422775~~ Support "run once" query evaluation over non well-behaving derived features
~~bug 398828~~ Duplicate extension entries in plugin.xml
~~bug 420303~~ Make the invocation of XBaseEvaluator.init() conditional
~~bug 425735~~ Query explorer does not refresh match descriptions on model changes
~~bug 423298~~ Query is not entirely updated when resource is reloaded (bug fixed, issue not closed yet)
~~bug 428015~~ IncQuery builder unnecessarily runs multiple times

Due to changes in GEF4 Zest API, versions 0.1.0.201310190205 or later are used for the extra, Zest-based visualizations (not included in the main update site)

Logger output for query-based features when query violates multiplicity contracts