panel-widget: don't use deprecated ...get_background_color
don't use deprecated gtk_cursor_new

https://git.gnome.org/browse/gtk+/commit/gdk?id=eedff394136013f31f9391e3c9d5b1ec5abc2c6f
panel-frame: don't use deprecated ...get_background_color
panel-properties-dialog.ui: fix deprecations
libpanel-applet: don't scale background
libpanel-applet: don't set background on invalid window
gnome-panel.css: remove box-shadow from menu bar
panel.c: don't crash clicking on panel handle

Documentation says that gtk_get_current_event can return NULL.
Also returned value must be freed.

https://developer.gnome.org/gtk3/stable/gtk3-General.html#gtk-get-current-event
applets: don't set background widgets

There is no need to set background widget for in process applets.
update background when PanelAppletFrame size changes

When PanelAppletFrame changes size update applets background. This
is possible fix for bug:
https://bugzilla.gnome.org/show_bug.cgi?id=500999
notification-area: don't use deprecated GdkColor
notification-area: don't use deprecated GtkAlignment
configure: bump required GTK+ version