better fade support for audio asset

<set/>, <play/> and <stop/> commands now support attributes 'fade'
(bool) and 'fadein'/'fadeout' (int, milliseconds time)

You can test it with something like this:
<show asset="tb_main" />
<line s="n">No music</line>
<set asset="music" track="test1" />
<line s="n">Music set</line>

<play asset="music" />
<line s="n">Music play</line>
<stop asset="music" />
<line s="n">Music stop</line>

<play asset="music" fade="false"/>
<line s="n">Music play</line>
<stop asset="music" fade="false"/>
<line s="n">Music stop</line>

<play asset="music" fadein="500"/>
<line s="n">Music play</line>
<set asset="music" track="test2" fade="false"/>
<line s="n">Music changed</line>
<stop asset="music" fadeout="500"/>
<line s="n">Music stop</line>

<set asset="music" track="test1" />

<play asset="music" fadein="500"/>
<line s="n">Music play</line>
<set asset="music" track="test2" fadein="500" fadeout="500"/>
<line s="n">Music changed</line>
<stop asset="music" fadeout="500"/>
<line s="n">Music stop</line>