Feature #65: --focus-exclude

- Add --focus-exclude, which should be used with a list of conditions to
  set certain windows to always be considered focused.

- Change focus determination process, with a newly added w->focused_real
  that reflects whether the window is actually focused, while w->focused
  represents whether compton considers it focused. The primary
  difference is now when a window considered focused because of
  --mark-wmwin-focused or --mark-ovredir-focused receives a FocusOut
  event, it's still considered focused by compton.

- Change opacity target and dim state calculation so that it's done at
  most once every paint.

- Split window opacity property fetching from calc_opacity() to a new
  function win_update_opacity_prop().

- Silence a warning in wid_get_prop_wintype().

- Rename a few functions. Code clean-up.

- My time is very limited currently, few tests are done, so this commit
  may very well introduce bugs.

- Known issue: Dim picture opacity does not change based on window
  opacity, causing somehow annoying effects when a window fades off.

compton.sample.conf

@@ -40,6 +40,7 @@ dbe = false;
 paint-on-overlay = false;
 sw-opti = false;
 unredir-if-possible = false;
+focus-exclude = [ ];
 
 # Window type settings
 wintypes: