Match selectors that combine an id with classes, such as p#i.c #527

Part of #520 (step 7).

p#i.c, p.c#i and #i.c never match. The . and # shapes in SelectorParser accept anything after the dot or hash, so p.c#i is stored as class C#I and p#i.c as id I.C: keys no element can have.

Fix.

• Parse an id with any number of classes, with or without a tag, as a simple selector and as a part of a descendant rule. Store them in one normal form, so p.c#i and p#i.c are the same key (for example P>>ID>>I>>CLASS>>C).
• Merge them after tag#id, in the simple groups and in the descendant sub-order.
• Tighten the . and # shapes to name characters, so a malformed part fails to parse and A descendant rule with a part mPDF cannot match is applied to the element its first parts name #521 drops the rule, rather than it becoming a key nothing matches.

Tests. p#i.c, p.c#i, #i.c and p.a.b#i, each alone and after an ancestor; each beats p#i and .c.

Depends on #521. Unblocks #538.