Feature #51: Re-initialize when SIGUSR1 is received

- Create a session_t structure, to get rid of most global variables and
  let information pass in/out mostly through parameters. Huge changes,
  so bugs may very well appear. I'm worried about resource leakages, in
  particular.

- Add gcc attributes on various functions.

- Add Doxygen configuration.

- Replace much Xlib Bool with C99 bool.

- Add and adjust some comments.

- Drop unused parameters from some functions.

- Cache default Visual and Depth, mainly to shorten code.

- Rename some types, variables, and functions.

- Add win_ev_stop() and set_ignore_next() for convenience.

- Modify wid_get_prop_wintype() and wid_get_opacity_prop() to use
  wid_get_prop().

- Rename --respect-attr-shadow to --respect-prop-shadow.

- Fix a memory leak in --respect-prop-shadow.

- Many other small changes.

.gitignore

@@ -34,7 +34,8 @@ install_manifest.txt
 
 # Misc files
 core
-oprofile_data
+oprofile_data/
 compton.plist
 callgrind.out.*
 man/*.html
+doxygen/