Clean up the FillToEdge action implementation

This extend the client_find_resize_directional() method to support
two growing modes, and return a bool for whether it was able to grow/shrink.

The client_find_resize_directional() method now takes an enum instead
of a bool, with two growing modes. The old mode which always tries to
grow, the a new mode that will only grow if the client's edge is not
already at a grow stopping point (ie against the edge of another window).