Change default icon size to 20 so they aren't defacto invisible

[ahydronous]

Some users their icons are "invisible" because the default size is set to 0. I chose size 20 because it makes most icons approximately the same size as the built-in top bar icons.

See: #382

[DanHolli]

size of zero is supposed to be an auto size as it functions in Ubuntu, it may be better to submit an issue and figure why yours isn't auto sizing

[3v1n0]

In theory the code at:

gnome-shell-extension-appindicator/appIndicator.js

Lines 1561 to 1569 in 63bed61

	if (sizeValue > 0) {
	if (!this._defaultIconSize)
	this._defaultIconSize = this._iconSize;
	this._iconSize = sizeValue;
	} else if (this._defaultIconSize) {
	this._iconSize = this._defaultIconSize;
	delete this._defaultIconSize;
	}

Should prevent this to happen, no?