Fixes 'cppcheck' warnings:

[libegg/eggdesktopfile.c:483]: (style) The scope of the variable 'try_exec' can be reduced.
[libegg/eggdesktopfile.c:483]: (style) The scope of the variable 'found_program' can be reduced.
[libegg/eggdesktopfile.c:484]: (style) The scope of the variable 'only_show_in' can be reduced.
[libegg/eggdesktopfile.c:484]: (style) The scope of the variable 'not_show_in' can be reduced.
[libegg/eggdesktopfile.c:486]: (style) The scope of the variable 'i' can be reduced.
[libegg/eggdesktopfile.c:608]: (style) The scope of the variable 'p' can be reduced.

[libegg/eggsmclient-xsmp.c:227]: (style) The scope of the variable 'cmdline' can be reduced.
[libegg/eggsmclient-xsmp.c:773]: (style) The scope of the variable 'fd' can be reduced.
[libegg/eggsmclient-xsmp.c:808]: (style) The scope of the variable 'keys' can be reduced.

[libegg/eggtreemultidnd.c:305]: (style) The scope of the variable 'context' can be reduced.

Fixes Clang static analyzer warning:

eggsmclient-xsmp.c:1156:18: warning: Casting a non-structure type to a structure type and accessing a field can lead to memory access errors or data corruption
    prop->vals = (SmPropValue *)vals->data;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~

eggsmclient-xsmp.c:1189:18: warning: Casting a non-structure type to a structure type and accessing a field can lead to memory access errors or data corruption
    prop->vals = (SmPropValue *)vals->data;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~

Replace G_DEFINE_TYPE_WITH_CODE with G_DEFINE_TYPE_WITH_PRIVATE macro

With minimal changes to avoid deprecated g_type_class_add_private

The environment variable XDG_SESSION_DESKTOP and DESKTOP_SESSION in
running_in_mate are compared using "MATE". Actually the environment
variable in MATE DE is "mate"(lower case). Using "mate" instead of
"MATE" to fix it.

Signed-off-by: Zhang Xianwei <zhang.xianwei8@zte.com.cn>