bump version to 3.15.0
clock: don't use deprecated GdkColor
clock: don't subclass deprecated GtkAlignment
clock: use g_strcmp0 not custom null_safe_strcmp
clock: don't use deprecated GtkAlignment
clock: don't use deprecated GtkMisc
clock: simplify size groups
libpanel-applet: fix critical warning
clock: properly handle visibility of edit button
clock: fix evolution opening from applet