[Trunk] Page settings

The new page settings dialog raised some questions here and here (in french).

I talked a little bit about this new "feature" with Werner. Having the same printable width for all pages is very nice for better performance. For example, if you resize the title frame vertically, you don't need to relayout all the music, but just compute the page breaks. The same for the preview btw, if you change the printable width everything need to be relayouted and the preview will be become slower with the number of pages, if you only change the current "margins", it's fast since you recompute page breaks only.

Still, it doesn't feel natural to me to set the printable width first and then "move its X coordinate" with the left margin. So I checked some software to see how they solved this issue. I checked in particular Lilypond , Sibelius, Finale & InDesign . Except Sibelius, none of them let you define totally different margins for odd and even pages. So they do use a fixed printable width area. But UI wise, they don't provide the "printable width" setting, they just let you set the margins and mirror them if you use twosided (or facing page) settings. In all software, Sib included, you can't choose different top and bottom margins for odd and even pages.

Attached a patch for a proposition according to this research. The printable width is gone but left and right margins change the printable width instead of the "coordinates". The text is gone and I used a logical layout instead. Top and bottom margins can be set separatly for odd and even, not sure if it's needed. In all cases, even if this proposition is not accepted, some compatibility code with 1.0 branch will be needed...