GladePreview: Use g_find_program_in_path() to launch the previewer.
Install widget icons into the hicolor theme.

Documentation seems to suggest that applications should
be installing their icons into the theme, we also no longer
call gtk-update-icon-cache at install time (we used to).

I made this particular change in order to cause the widget
icons to show up properly in bundles, if this commit is wrong
then we need to figure out how it actually works that icons
show up in the palette (so we can reproduce that in the bundle).
Added GLADE_BUNDLED environment variable.

This environment variable is used to detect a bundled environment
for which Glade will run on various 64bit linux systems.

Currently we just avoid loading any catalogs except for the gtk+.xml
catalog in the case of a bundled environment (but there may be
some extra special cases to add, in order to ensure we don't integrate
with the unknown running system).
Added new linux64 build subdirectory.

This build subdirectory includes instructions and resources for
building bundles which are relatively portable for any 64bit linux
distribution.