Update maven mirror generator action.

[mbien]

• None of the preset mirrors for maven central exist anymore, this updates the preset to the last known public mirrors and tweaks the UI a bit.
• Fixed possible NPE when the central list is checked before NotificationLineSupport was attached.

test:
open settings.xml via Project Files, hit alt+insert -> Mirror...

[mbien]

I asked on the apache slack maven channel about an updated list of public mirrors which caused a little discussion between maven devs and also central maintainers. This lead to an update of the meta xml, which is now listing the remaining public mirrors correctly:
https://repo.maven.apache.org/maven2/.meta/repository-metadata.xml

So we could collect the mirror info from the repo xml, but since its just 3 urls it is probably ok to keep them hardcoded for now without over engineering it.

[mbien]

would like to start to merge some of the smaller maven PRs this weekend, assuming they are reviewed to shrink the queue a bit.

[matthiasblaesing]

Looks sane to me.

[mbien]

@sdedic to answer your question from the other PR, the NPE was here:

netbeans/java/maven.grammar/src/org/netbeans/modules/maven/codegen/NewMirrorPanel.java

Line 167 in a9845e3

nls.setErrorMessage(NbBundle.getMessage(NewProfilePanel.class, "ERR_SameMirrorId"));

nls was null. This can happen when the mirror already exists and the user opens the dialog again, the check will run right from the constructor, nls is set post-construction and is therefore still null. By moving the check into addNotify() this is avoided.