I10N: First translation fixes for 2.1

Fix README, some spelling errors, remove contractions,
exclamation marks, correct some plural forms, join split messages.
Add QCoreApplication::translate to non-Q_OBJECT-classes.
Fix some lupdate warnings about discarding meta-data (//:).

src/plugins/projectexplorer/buildstepspage.cpp

@@ -259,7 +259,7 @@ void BuildStepListWidget::stepRemove(int pos)
     } else {
         QMessageBox::warning(Core::ICore::instance()->mainWindow(),
                              tr("Removing Step failed"),
-                             tr("Can't remove build step while building"),
+                             tr("Cannot remove build step while building"),
                              QMessageBox::Ok, QMessageBox::Ok);
     }
 }