Use xdg-open by default in mc.ext.in if present to open files, fallback on current scheme otherwise

[zaytsev]

There've been a huge number of tickets requesting mc.ext.in modifications, some of which are not fixed yet:

#1800
#1721
#1686
#1452
#2117

Some of these changes are conflicting and different users prefer different associations, so controversy is inevitable.

I suggest to change the current logic to a more versatile: if xdg-utils are present on the system ( ​http://portland.freedesktop.org/wiki/ , all modern Linux distributions, such as Ubuntu / Debian, Fedora, RHEL etc.) try to use xdg-open to open the file. Otherwise fallback on the traditional associations scheme.

xdg-open uses mime-type associations set by the user to start the appropriate application, therefore, one might use his desktop controls to centrally manage file associations at will.