core: calculate fontRatio and apply it in GCText

On some low-dpi Android devices with high res (e.g. Galaxy Tab 4) the
fonts in Text-like elements appear too small with respect to the other
graphics -- also if we are using font.pointSize.

For these cases we calculate a fontRatio in ApplicationInfo that takes
dpi information into account, as proposed on
http://doc.qt.io/qt-5/scalability.html#calculating-scaling-ratio

GCText applies this factor automatically on its new fontSize property.
Menu: use device independent font-size
core, NumPad: use GCText and device independent font-size
core, GCText: add note about pixelSize usage
core, Score: rename fontSize property and use device independent
font-size
core: use device independent font-size
activities: use device independent font-size
core, GCText: add another note
braille_alphabets: use GCText and device independent font-sizes
core, template: use GCText and device independent font-sizes
clockgame: need to clear pointSize when using pixelSize
scalesboard: use GCText and device independent font-size