evdev-touchpad: Set some options using weston.ini

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.
diff --git a/shared/config-parser.h b/shared/config-parser.h
index 794e09c..410a7ef 100644
--- a/shared/config-parser.h
+++ b/shared/config-parser.h
@@ -88,6 +88,10 @@
 			       const char *key,
 			       uint32_t *value, uint32_t default_value);
 int
+weston_config_section_get_double(struct weston_config_section *section,
+				 const char *key,
+				 double *value, double default_value);
+int
 weston_config_section_get_string(struct weston_config_section *section,
 				 const char *key,
 				 char **value,