build: corrected installed vs uninstalled behavior

Relying on the binary name to discriminate between installed and
uninstalled behaviors. This implementation has been succesfully tested
in the ntdisp project: check http://dev.entidi.com/p/ntdisp/issues/95/
for technical details.

Closes issue #94: http://dev.entidi.com/p/adg/issues/94/
build: added installer phony target
demo: relocated the default window icons

Instead of hard-coding PKGDATADIR in libadg, the function is now generic
and the path decision is demanded to the application (that has access to
its arg[0]). Modified and adapted the sources to properly handle the
icons and the .ui files.

Put some shared code in demo.[ch] to avoid duplication.
demo: revamped GTK+2 port
build: use a proper $INSTDIR on win64 installer

By default, nsis installs under "Program files (x86)" on win64 platform
while the program, being a native win64 binary, must be installed under
"Program files". Modified NSis script accordingly.