Set Mscore::DPI from logical DPI not physical

[softins]

Currently, when editing chord names, they appear larger than when not in edit mode. This appears to be due to Mscore::DPI having been set to the physical DPI of the screen, but the QTextDocument uses the logical DPI.
Setting MScore::DPI to the logical DPI fixes the change of size when editing chord names. It also appears to make MuseScore correctly obey the user's display scaling set in the Windows control panel.

This change hasn't been tested on Mac or Linux, but it seems from the comment that DPI should be set from the logical DPI anyway.

[Jojo-Schmitz]

See also http://musescore.org/en/node/23051

[MarcSabatella]

I have no idea what the ramifications of this would be in general, or how common the issue of different physical and logical DPI values (as reported by Qt) is. But given things are not great for some of us now, it seems it would make sense to try this out and see if it makes things worse for anyone in any way. If so, then maybe roll it back, otherwise leave it in.

[softins]

Well on my Windows 7 with external monitor, the debug output from MuseScore reports 72dpi (approx) as the physical DPI, but the logical DPI depends on the display scaling set in the control panel. On my system it is 96dpi when at 100%, 120dpi when at 125%, and 144dpi when at 150%. Standard apps will scale themselves dependent on the logical DPI. Without the above change, at least some parts of MuseScore, including the score, do not scale (I haven't checked exhaustively), but the QTextDocument does scale automatically. As QTextDocument is used in edit mode only, for chord symbols, that is why edit mode can be a different size from the rendered symbols.

[lasconic]

On MacOSX some texts (Instrument names in particular) appears smaller than usual. With this pull request, these texts looks better. I'm afraid that something changed between Qt 4 and Qt 5. Apparently in Qt5 the logical DPI is somehow used to scale the texts.

On a related matter, PDPI is never used in the code as far as I can tell. Only DPI is used.

[lasconic]

Let's try it on a larger scale.