Add alternative way to create id for Qt .ts files

Bug reports
#1

Please consider creating an alternative way to set an ID from a .ts file, as Qt will deprecate .ts files with both id= attribute and … from Qt 6.10 onwards.

h2. Current behavior

Qt up until now allows you to create .ts files with following structure:

   <context>
          <name>MyContext</name>
          <message id="prefix">
               <source>source</source>
       ...

I understand localazy will import this as ‘prefix.myid’ with source string ‘source’, and some users of both Qt and localazy do rely on this.

In Qt, we now want to deprecate this behavior though, as it is mixing the concepts of id-based and source-string-based translations, see https://bugreports.qt.io/browse/QTBUG-139769. This means that files in future will have either an ‘id’ attribute or a ‘context’, not mixing both.

h2. Suggested alternative

To allow people to migrate, it would be nice if localazy would accept following structure:

   <context>
          <name>MyContext</name>
          <message>
               <source>source</source>
               <extra-localazy-prefix>prefix</extra-localazy-prefix>
       ...

This would allow existing users to keep using their translations.

Thanks for considering, the maintainer of Qt Linguist.