Add 64 and 256 icon sizes to geany.ico

On Windows with HiDPI screen the Geany ico is really blurry and hopefully
this fixes it. The icon was generated with the following commands:
```
inkscape -w 64 -h 64 -e 64.png scalable/geany.svg
inkscape -w 256 -h 256 -e 256.png scalable/geany.svg
convert 256.png 64.png 48x48/geany.png 32x32/geany.png 16x16/geany.png geany.ico
```
(requires inkscape and imagemagick installed)

Based on

https://docs.microsoft.com/en-us/windows/win32/uxguide/vis-icons

Microsoft recommends 16, 32, 48, 256 sizes of ico files and 64 for
"classic mode" so the generated sizes should cover all of them.

I didn't make a Windows build with this icon so I actually haven't
tested it myself if it fixes the problem.