build: build flatpak with new --with-icc-directory option.
This will set the default color profile directory under its path inside
the sandbox and therefore fix issue #15 of our flathub repo.
While I am at it, I do some cleaning and remove --enable-vector-icons
since this is now the default.
This is in particular necessary because permission in manifest
--filesystem=/usr/share/color/icc does not work as expected to mount the
system repository inside the sandbox at the same path.
See commit
8f3bf7b175d44118f9146e31191fcb7558614b31 from flathub
repository for org.gimp.GIMP.