Process group cycling

- Created a function to cycle through windows from processes in the same
  group. For example, while focused on a browser window, pressing Caps+,
  will swap out the currently focused browser window for another browser
  window without changing the currently selected tags in a manner
  similar how the "next" command cycles through windows in GNU screen.
Use inotify to monitor mail folders

- dwmstatus uses inotify to check for changes in mail directories
  instead of periodic polling.
- The Makefile target for dwmstatus has the source code as a dependency
  so "make" recompiles dwmstatus only when the source code has changed.