various improvements to `px:mediaoverlay-rearrange`

 * if a text chunk is associated to several audio clips, the resulting
audio clip will span from the earliest `clipBegin` to the latest
`clipEnd` (in `rearrange.xsl`).
 * `generate-ids.xsl`,`rearrange.prepare.xsl`,`rearrange.xsl` : slightly
improve the XSLT code style ("push" approach)
 * remove `rearrange-library.xpl`