cmd_client_*: refactor duplicated code

This is the second in a series of commits to refactor the color handling
in sway. This removes the duplicated color parsing code in
sway/commands/client.c. Additionally, this combines the parsing of
colors to float arrays with that in sway/config.c and introduces a
color_to_rgba function in commom/util.c.

As an added bonus, this also makes it so non of the colors in a border
color class will be changed unless all of the colors specified are
valid. This ensures that an invalid command does not get partially
applied.

include/util.h

@@ -17,6 +17,8 @@ int wrap(int i, int max);
  */
 bool parse_color(const char *color, uint32_t *result);
 
+void color_to_rgba(float dest[static 4], uint32_t color);
+
 /**
  * Given a string that represents a boolean, return the boolean value. This
  * function also takes in the current boolean value to support toggling. If