Make windows scalable by DPI.
Fix poorly named variable

If a variable holds scaling factor, it shouldn't be named "dpi" - makes
no sense and is highly misleading.
Better way to get screen's DPI
Use a nicer way of expressing pixel sizes

Instead of multiplying by scale factor in every place pixel values are
used, use a helper macro PX() that does the job.
Use PX() for window padding too

0b71dcee didn't use HiDPI scaling for the main window's padding, so fix
that.
Use high-rest icons when appropriate

Use LoadIconWithScaleDown() (dynamically loaded so that the code works
on XP) when appropriate to load higher-resolution icons when running
under HiDPI.
Update stock icon with higher-resolution sizes

Update software-update-available.ico (used for frame's icon and as a
fallback for app icon) with larger sizes to look nice in HiDPI. This
increases WinSparkle.dll size somewhat, but that's a necessary price to
pay.
Merge branch 'hidpi-fixes'

Fixes #36.