This pull request basicly adds the suport for gtk3 dialogs to she.
it is disabled by default and can be enabled with
cmake ... -DWITH_GTK_FILE_DIALOG_SUPPORT=ON ..
and needs the Experimental option "Use native file dialog"
enabled in aseprite enabled.
It is currently only available on *nix and has a external
dependencie on gtkmm >= 3.16 and glibmm >= 2.45
To have at least backwardcompatibility with ubuntu 15.10
the cmake option -D_WITH_DEPRECATED_GLIB_SUPPORT was added
to make use of a deprecated glibmm method.
Further testing ist need to see how far this backward
compatibility works, as of now ubuntu 12.04 does not seem to work.
But as ubuntu 12.04 will reach its end of support this summer and
ubuntu 15.04 having passed its end of life in february the question
is how far back this support is aimed for. Please enter the commit message for your changes. Lines starting
