Ignore reAddon plugin prefix values format for text attribute

[beverlyck]

When we modify update or set the text attribute, the "snap.util.attr" event triggers and that causes a chain of callbacks for the event. If the text value starts with with a prefix format like here,

Snap.svg/src/svg.js

Lines 1399 to 1401 in 6f02a26

	* Prefixed values in format `"+=10"` supported. All four operations
	* (`+`, `-`, `*` and `/`) could be used. Optionally you can use units for `+`
	* and `-`: `"+=2em"`.

For example, + = 3 will throw an error on

Snap.svg/src/attradd.js

Lines 40 to 63 in 6f02a26

	eve.on("snap.util.attr", function (val) {
	var plus = Str(val).match(reAddon);
	if (plus) {
	var evnt = eve.nt(),
	name = evnt.substring(evnt.lastIndexOf(".") + 1),
	a = this.attr(name),
	atr = {};
	eve.stop();
	var unit = plus[3] || "",
	aUnit = a.match(reUnit),
	op = operators[plus[1]];
	if (aUnit && aUnit == unit) {
	val = op(parseFloat(a), +plus[2]);
	} else {
	a = this.asPX(name);
	val = op(this.asPX(name), this.asPX(name, plus[2] + unit));
	}
	if (isNaN(a) || isNaN(val)) {
	return;
	}
	atr[name] = val;
	this.attr(atr);
	}
	})(-10);

This PR added a check, if the reAddon matcher is successful and if the attr name is "text", it returns instead of stopping propagation and performing the prefix value operations.

[beverlyck]

The reAddon plugin change here but when I rebuilt from the source, the other files were changed.