Handle all pixel values in XRC as being DPI-independent.

Pixel values in XRC can never be correct for high resolution displays, unlike
the pixel values passed to wxWidgets API, which could be already adjusted to
account for the resolution or obtained from resolution-dependent text metrics,
so scale them by the factor appropriate for the current resolution
automatically.

docs/changes.txt

@@ -37,6 +37,9 @@ Changes in behaviour which may result in build errors
 3.1.0: (released 2014-xx-xx)
 ----------------------------
 
+- Many improvements for high DPI monitors support, notably XRC now interprets
+  all pixel values as being in resolution-independent pixels.
+
 - wxQt branch implementing Qt5-based port of wxWidgets API was merged into
   the trunk (Mariano Reingart, Google Summer of Code project).