Finish porting to xfconf

Porting to xfconf created duplicates of all configuration options. In other words, all options were being stored at two locations in memory when the plugin was running. This patch removes most of the duplicates which simplifies the source code and makes it easier to apply changes made by the user in the properties dialog.

This patch removes struct t_monitor_options, and removes fields from structs t_global_monitor and t_uptime_monitor. The only redundant settings left to be removed by a future patch are timeout & timeout_seconds in t_global_monitor.

Additionaly, it fixes memory leaks created by g_value_dup_boxed().

Configuration update is now not emitting a change signal if the new color selected by the user is the same color as the current color.