add custom folding regions

[danthe1st]

Relevant issues: https://bugs.eclipse.org/bugs/show_bug.cgi?id=173796 (and there are also similar ones like https://bugs.eclipse.org/bugs/show_bug.cgi?id=258965 for CDT)

What it does

This PR allows to add custom folding regions using comments.

I added new options to the preferences in Java > Editor > Folding (not that I changed this preference page to use Groups):

If a comment containing the start region and a comment containing the end region are present in the same source file, a folding region is added allowing to collapse that section.

This works even for elements of different levels, e.g.

// #region
void a(){
    // #endregion
}

Notes / What I'm not sure about

Custom folding regions are disabled if either of the custom texts/preferences are empty or if one contains the other. Should I add some information about that to the preferences? Would it be better to disable it by default (currently, it defaults to #region/#endregion)?

I haven't written any tests so far. Are there existing tests for folding in JDT-UI? What about performance tests (my implementation requires scanning more parts of the source file which could affect performance)?

Regarding performance, I used String#contains to check whether comments contain certain the region begin/end tokens (e.g. #region/#endregion) which requires creating a String for each comment every time the file is edited. Should I change that to use the char[] returned by getCurrentTokenSource() or similar?

I have bumped the minor version of org.eclipse.jdt.ui due to adding new elements in PreferenceConstants. Is this ok that way?

I guess this should be added to the "New and noteworthy" list (assuming this change gets accepted and merged). If so, should I add it and is this the right place?

How to test

• Create a Java source file
• Create a comment containing the text #region
• Create a comment containing the text #endregion
• Test similar things with the source file
• After editing the corresponding changes in Java > Editor > Folding, it is necessary to reopen the source file.

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[HannesWell]

@fedejeanne in the dev-call on Thursday you mentioned that you could have a look at this, does this still stand?
Eventueally we need a JDT committer to submit this anyways.