Take wxBitmapBundle instead of just wxBitmap in various public functions
and select the bitmap appropriate for the current DPI scaling when we
need to use it.
Note that this removes wxAuiScaleBitmap() which used custom scaling
logic that might be better than the generic approach used by
wxBitmapBundle, and so could result in a temporary regression in the
icons appearance in high DPI, but they will hopefully be replaced with
SVG versions soon, so that this shouldn't matter.
This wxAuiTabArt implementation uses Windows uxtheme API to draw AUI tabs. A
fallback to generic tab art is implemented for the bottom tabs or if uxtheme
is not available (disabled by user or no manifest file).
Closes https://github.com/wxWidgets/wxWidgets/pull/105
