XML format: add support for multiple keys per element

[kairos10]

It seems that the current parser does not support multiple keys for a single definition.

<entry><key>form1</key><key>form2</key><definition>some definition applicable for both keys</definition></entry>

[pettarin]

Indeed. The DTD of the (Penelope-defined) XML format is https://github.com/pettarin/penelope/blob/master/penelope/res/dictionary.dtd

In view of https://github.com/pettarin/penelope#important-update I leave the issue open, but I do not plan to address it. You can workaround this issue by creating two <entry> elements with the same <definition> but different <key> values.