freekdesktop.org says all themes must fall back to 'hicolor' if they don't find

freekdesktop.org says all themes must fall back to 'hicolor' if they don't find
a theme-specific icon (and as such hicolor is where applications should
install their icons).  So: don't install some of our icons in the gnome area,
install them all in hicolor.

While we're at it, go ahead and install all the icon sizes we have.

If we're on SuSE, use their desktop-file-updater macro; without that they
won't recognize our desktop file.

Fix bug which prevented the MIME database from being updated if our install
prefix is not /usr .

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@48204 f5534014-38df-0310-8fa8-9805f1628bb7