Remove <requires> from installable features

They're useless and they only caused troubles in the past, see #20 and
#21 

Change-Id: I2e9e3fc01bb08eb49d5decf183fd1f1f90048caa
Another attempt with waitForNodes

Change-Id: Ice11dc789ec7e3a72c65ca4248d17250201ae683
explicit Xtext dependency ranges in main bundles

Change-Id: I7f9f44a293d88dfc2f0366761e78dde48881d07c
increment versions in pom.xml and set xtext versions

Change-Id: I21fb01f0ed15015679a3c4910fbcee70c7cfc81d
Incremented version 1.5.1

Change-Id: I65d2a1ad2f27b67a3b4e40abbc9527fac23c84a6
project settings for Sonar

Change-Id: I5c1d8f3491af2fd9649fdca8ee9ed251997d77da
Updated api_filters

Change-Id: Iff26e1c358325253a3e687e862e4e276e82f2370
RuleFailedException made immutable

Change-Id: Ie931ad5ffdd4843e490ec53aeae49ccf89483320
Fixed some naming conventions

Change-Id: Ia719c577e642d1cc6115a2e4ff4a29c0a022b12c
removed Activator from it.xsemantics.runtime

it was useless

Change-Id: I167fdfdb7f9e51ccbde84fa82dc56a921558e387
Some cleanup in StringRepresentation

Change-Id: Ic60d08d95841b819ffbd97988422e73a83bd2ece