Usability: Taxonomy edit term

Taxonomy usability has been improved a lot since D5. Especially Dries' ingeniously simple idea to just remove a good part of the UI cruft and let the taxonomy backend handle hierarchies etc. made things easier (see #192242)

Still things are not yet optimal. I can see the following problem:

• There is no help text for "Parents" and "Related terms" at all. Plus: The help text for "Synonyms" doesn't explain the concept. This is a shame, as these concepts are not necessarily self-explaining and bring up a lot of support questions.
• While parent terms are used in vanilla D6, both related terms and synonyms are not: They are just collected to be used by some contrib modules. This is hard to explain.
• The parent terms selector is shown even for necessarily flat "free-tagging" vocabularies. Changing parent terms however doesn't do anything. This is unnecessarily confusing.

Now, "Parent terms" is not really an advanced feature, it is needed for all but freetagging vocabularies. The big difference to "Related terms" and "Synonyms" is that "Parent terms" can be used rightaway, while the others are contrib-only. For sake of usability it is important to draw the line between settings people may use rightaway and settings to be used only if told so.
Hiding "Parent terms" in the "Advanced settings" fieldset means many people will use that advanced setting, and then many people will play around with the other two advanced settings as well. Then people will post support questions why synonyms don't work etc.

For these reasons, the better solution would be to completely hide the "Parent terms" selectbox for free-tagging vocabularies, but move it up to the general settings for regular vocabularies. Of course we should add a short help text, and yes this should be a rare exception to the string freeze IMO (one new string, another one is being reused).
"Related terms" and "Synonyms" as well as the "Weight" would stay in the collapsed "Advanced" fieldset. The "Identification" fieldset was removed as it wouldn't make sense anymore and was removed.

I also fixed _taxonomy_term_select to support flat vocabularies. Before it always returned a hierarchical map of terms, even for flat free-tagging vocabularies, which is pretty confusing.

In D7 we could further improve help texts, but for now this should be quite a step forward.