build: Rename the names of the icons' files

The names of icons' files, besides containing the final names, they
also contained the path in which they were going to be installed.
This implies that they have to be renamed on the installation
process.

This patch creates the proper directory tree needed on the
installation process and renames all files' names to their final
names.

https://bugzilla.gnome.org/show_bug.cgi?id=787013