[dev] [dwm] visible only if needed [patch]

Code from Hiltjo Posthuma and Jeremiah Dow ( with minor add ) is merged in this patch.
The behaviour is :

- border is shown when one sees more than one client.
- tags are shown when they are viewed or when clients are labelled with them.

drawsquare is removed.

As i don't know C, please say anything i could have messed up
while playing with the code.