Switch to using float for point size in wxNativeFontInfo

Use float as the fundamental type for the font size and implement
wxNativeFontInfo::{Set,Get}PointSize() as wrappers around the new
{Set,Get}FractionalPointSize().

Update wxNativeFontInfo for all platforms and replace the use of its
SetPointSize() method with SetFractionalPointSize() in wxFont for the
platforms already supporting fractional point sizes (don't change the
others just yet).

Note that wxNativeFontInfo::{Get,Set}PointSize() are preserved for
backwards compatibility but shouldn't be used in any code inside the
library itself any more (again, this is not the case yet, but will be
soon).

src/qt/font.cpp

@@ -315,9 +315,9 @@ void wxNativeFontInfo::Init()
 {
 }
 
-int wxNativeFontInfo::GetPointSize() const
+float wxNativeFontInfo::GetFractionalPointSize() const
 {
-    return m_qtFont.pointSize();
+    return m_qtFont.pointSizeF();
 }
 
 wxFontStyle wxNativeFontInfo::GetStyle() const
@@ -412,9 +412,9 @@ wxFontEncoding wxNativeFontInfo::GetEncoding() const
     return wxFONTENCODING_MAX;
 }
 
-void wxNativeFontInfo::SetPointSize(int pointsize)
+void wxNativeFontInfo::SetFractionalPointSize(float pointsize)
 {
-    m_qtFont.setPointSize(pointsize);
+    m_qtFont.setPointSizeF(pointsize);
 }
 
 void wxNativeFontInfo::SetStyle(wxFontStyle style)