Cleanup direct use of `new HtmlTree`

For clarity of code and ease of reading, we have tended towards using factory methods to create instances of `HtmlTree`, using `HtmlTree.TAG(...)` instead of `new HtmlTree(HtmlTag.TAG`.

Auditing the remaining direct uses of `new HtmlTree` reveals that many can now be replaced by existing factory methods, and that only a few factory methods need be added to completely transition to this code pattern.

Here are the stats for the remaining non-local uses of `new HtmlTree`:

$ grep -r -o 'new HtmlTree(HtmlTag[^)]*)' open/src/jdk.javadoc | grep -v HtmlTree.java | sed -e 's/^.*://' | sort | uniq -c | sort -h -r
   7 new HtmlTree(HtmlTag.HR)
   7 new HtmlTree(HtmlTag.BR)
   6 new HtmlTree(HtmlTag.WBR)
   6 new HtmlTree(HtmlTag.BUTTON)
   5 new HtmlTree(HtmlTag.CODE)
   4 new HtmlTree(HtmlTag.DIV)
   3 new HtmlTree(HtmlTag.PRE)
   3 new HtmlTree(HtmlTag.LI)
   3 new HtmlTree(HtmlTag.DD)
   3 new HtmlTree(HtmlTag.BODY)
   2 new HtmlTree(HtmlTag.UL)
   2 new HtmlTree(HtmlTag.IMG)
   1 new HtmlTree(HtmlTag.SCRIPT)
   1 new HtmlTree(HtmlTag.P)
   1 new HtmlTree(HtmlTag.META)
   1 new HtmlTree(HtmlTag.LINK)
   1 new HtmlTree(HtmlTag.HEAD)
   1 new HtmlTree(HtmlTag.EM)


Analyzing those results further, we can identify which tags have like-named factory methods, and which do not:

HR:
BR:
WBR:
BUTTON:
CODE:
    public static HtmlTree CODE(Content body) {
DIV:
    public static HtmlTree DIV(HtmlStyle style) {
    public static HtmlTree DIV(HtmlStyle style, Content body) {
    public static HtmlTree DIV(Content body) {
PRE:
    public static HtmlTree PRE(Content body) {
LI:
    public static HtmlTree LI(Content body) {
    public static HtmlTree LI(HtmlStyle style, Content body) {
DD:
    public static HtmlTree DD(Content body) {
BODY:
UL:
    public static HtmlTree UL(HtmlStyle style) {
    public static HtmlTree UL(HtmlStyle style, Content first, Content... more) {
    public static <T> HtmlTree UL(HtmlStyle style, Collection<T> items, Function<T,Content> mapper) {
IMG:
SCRIPT:
    public static HtmlTree SCRIPT(String src) {
P:
    public static HtmlTree P(Content body) {
    public static HtmlTree P(HtmlStyle style, Content body) {
META:
    public static HtmlTree META(String httpEquiv, String content, String charset) {
    public static HtmlTree META(String name, String content) {
LINK:
    public static HtmlTree LINK(String rel, String type, String href, String title) {
HEAD:
EM:

Thus, the following tags do not have factory methods:

HR, BR, WBR, BUTTON, BODY, IMG, HEAD, EM

Also of note, HR, BR, and WBR could use singleton instances.