GSettings: Added schema file.

This one was converted from the GConf schema file and will replace it.
GSettings: Added compilation and installation of schema file.
GSettings: Schema empty values should be empty.
GSettings: Fixed schema compliance.
GmMenuAddon: Dropped obsolete settings.
INSTALL: New version.
GmMenuAddon: Removed unused function.
GSettings: Added TODO. Not to forget.
GSettings: Ignore GmConf ugprades.

We are starting from fresh schemas.
GSettings: Fixed schema to add missing values.
GSettings: Added initial implementation of GSettings.

It is still working similarly to GConf. But the road to replace
everything to a pure GSettings port will be rather long. Having a GmConf
implementation is probably the safest thing for now.

Notifiers are not reimplemented yet.
GmMenuAddon: Dropped reference to removed function.
GSettings: Converted GmWindow objects to use GSettings.
GmDialog: Removed antic and unused functions.

Those date back from the good old GnomeMeeting days!
GmPreferences: Added missing config.h.
GmWindow: Removed useless public functions.

Those were left over from a previous commit.
GSettings: Do not remember positions for the prefs and assistant.

Those were never remembered. The problem with the assistant, for
example, is that it derivates from GtkWindow, and it is impossible to
make it derivate from our GmWindow object. Remembering position and size
is probably too difficult for those.

Please note that it is the WM job to remember those settings. However,
we have our own code for non-Linux ports.
Code simplification and cleanups.

This is required for commit 9459a20e34515cf9d4558246f32e3a3091df5c5a.
GSettings: Ported the sound events part of the preferences window.
GmPrefs: Fixed sound event playing if full filename is provided.

This probably also affects 4.0.x.