Enhancing support for multiple outputs

Nowadays, more and more musicians use tablets instead of paper.

For the editor, this leads to think on different layouts depending on the output media.

For example, the notion of "page" is very relative on tablets and is even annoying when using a auto-scrolling PDF reader.
For bands, you often edit parts in landscape mode in little format.
All this is well covered with stylesheets.

Currently you can produce two sets of parts: one layered for paper, the other layered for tablets.
This leads to some problems:
- it's no more possible to apply a style to "All parts" because you want to apply it only to one set or another one.
- for the tablets, you have to fix the height of the "page" which doesn't have really a signification.
- You end up with tons of parts, each for a different layout

It'll be nice if you could have simply multiple stylesheets applicable on a single part and have all of them generated to PDF.
This will probably need to keep the information of the applied stylesheets somewhere, which is currently not the case.

Sorry, the example is big, but I've found no shorter way to show the problems (and it's only a short 5tet score - imagine for a 25-part symphony ;-)))