Moving validation to common

Copied package/module name validation function from ide-eclipse to ide-common.

The ``Escaping`` class was needed by the new ``NameValidator`` and for this reason was also copied into ide-common: https://github.com/ceylon/ceylon-ide-eclipse/blob/master/plugins/com.redhat.ceylon.eclipse.ui/src/com/redhat/ceylon/eclipse/util/Escaping.java (which makes sense as this will probably be used by IntelliJ as well).

Unfortunately, I have not been able to find the tests for ``Escaping``. Surely it must have some tests, right?

So [``Escaping``](https://github.com/ceylon/ceylon-ide-eclipse/blob/master/plugins/com.redhat.ceylon.eclipse.ui/src/com/redhat/ceylon/eclipse/util/Escaping.java) and [``packageNameIsLegal``](https://github.com/ceylon/ceylon-ide-eclipse/blob/master/plugins/com.redhat.ceylon.eclipse.ui/src/com/redhat/ceylon/eclipse/code/wizard/NewUnitWizardPage.java#L714) may be removed from the ide-eclipse project after this PR is merged.

The moved class and function moved into ``ide-common`` can then be used in both Eclipse and IntelliJ (see issue https://github.com/ceylon/ceylon-ide-intellij/issues/47).