Solidify .ico image format support

The MS .ico format is not widely used. However, it is an absolute requirement for webkit, to handle favicons. Hence, it is not attractive to move the ico handler code over to the qtimageformats module, since that would create an unwanted dependency on that module.

There are some issues with current implementation in qtbase, however:
1) It is implemented as a plugin, but without any configure options - it is always built.
2) It contains a redundant implementation of bmp decoding

Since the .ico format is actually just a thin container for a set of bmp and/or png images, the task would be to
a) Reimplement a qicohandler.cpp so that it uses (private api of) qbmphandler.cpp and qpnghandler.cpp for the actual image decoding. (I.e. a far more light-weight implementation).
b) Place that into src/gui/image and code it into qimagereader.cpp etc. as a built-in format. (i.e. drop the whole plugin thing.)

Writing support is not important. Multiple images in file should be available using QImageReader's imageCount(), jumpToImage() etc.
.CUR support is nice but optional.