Remove default values from RC configuration files

This simplifies updates over time. That is: users who are using default values will automatically receive the updated values when switching from xfce4-sensors-plugin version N to version N+1.

Changes:

• Remove sensors_init_default_values() -> Add t_sensors constructor
• Specify default values in 1 place (t_sensors constructor), instead of 2 places (sensors-interface-common.cc, configuration.cc)
• Add xfce4::Rc::write_default_... functions
• Add support for floating-point values (32-bit) to xfce4::Rc
• Add parse_float() and parse_double()

Fixed issues:

• Save the current width&height values (not the stale values) to RC file
• Rc::read_entry() fallback code-path

See also: #26 (closed)