Implement window gravity in ConfigureRequests (FS#1137)

Previously, awesome didn't move windows when they sent a ConfigureRequest (and
it also shrunk them by the size of titlebars, before it forgot to add these).
This commit adds proper gravity support to ConfigureRequest handling.

This was tested with test-gravity from metacity (in src/wm-tester).

Signed-off-by: Uli Schlachter <psychon@znc.in>