This patch adds 3 new options to weston.ini to allow
the user to change default constant_accel_factor,
min_accel_factor and max_accel_factor. If no options
are set, it falls back using defaults as it did before.

v2: create weston_config_section_get_double and use it
instead of manualy converting string to double.

v3: add default values in weston_config_get_double
instead of using conditionals.

v4: don't pass diagonal as pointer.