trim bundled icons

[Antoine Martin]

We reduced the size of the win32 builds in #2550 but thanks to r25081 (for CSD and windows with a header bar), we've gained more than we had lost!

Find a way to trim the icons we bundle, and more importantly the metadata that goes with it.

[totaamwin32]

Done in r25141.

We've shaved off over 10MB of icon data we don't need and more importantly countless files which slowed down the installer.

[Antoine Martin]

(this also fixed the missing theme icon: #2531)