We need to check the existence of dconf-editor's GSettings schema before
using it, because the program will abort if dconf-editor is not installed
on the system when it tries to access it.

Mostly found by cppcheck.

origin commit was:
https://github.com/mate-desktop/mate-panel/commit/96c7ebc

This reverts commit 96c7ebc6dc9e8b8327db04a9570054ee78743353.

Mostly found by cppcheck.

- Build the list in reverse order, then reverse the result.  This is
  useful because GS?List are list nodes, not containers of nodes, and
  thus don't contain a pointer to the list's end, meaning to append one
  has to walk the entire list to find the end each time.  To avoid this
  we use the common idiom of prepending to the list (which is cheap, as
  it's adding a node before the given one), and then reversing the
  resulting list to get back the original order.
- Avoid unnecessary memory copy by stealing the GStrv's members.  We
  get the array as a copy, so we can simply steal the members and free
  the container array only, saving a copy for each member.

* tell dconf-edit about relocatable schemas

* self fix for incorrect and duplicated records

remove duplicated schemas for currect path
remove incorrect schemas for current path

* remove clock applet hack

* expose mate_panel_applet_settings_new to introspection

add "transfer full" annotation to make this method available throught
introspection