Add icon support

- icons are displayed at the top left corner
- disabed by default, fully backwards-compatible
- no additional dependencies, only cairo used
- TODO: only supports png (but all default images are png)
- TODO: message formatting is sometimes buggy
Fix stuff regarding icons

- fix icon_path string calculations
- add default settings to config.def.h
Add option to align icons left or right
Change default icon size to 16x16 (was 32x32)

might fit better to dunst's minimalistic style
Fix error on non-existent icon names
Change how icons are loaded

- use several icon_folders instead of a single icon_path
- check all paths for the icon
Introduce default icon setting
Add script option to override shown icon
fix memory leak in string_to_argv
Fix layout alignment with dynamic width.

Update pango layout's width if width has changed and alignment != left.
Set _NET_WM_STATE_ABOVE and fix window type.

- Fix indentation.
- Fix wrong paramenters in call to XChangeProperty
- Do not try to raise the window, set _NET_WM_STATE_ABOVE instead
Add 'msg_urgency' as a new filter to match on the urgency.

This patch adds a new filter 'msg_urgency' which _matches_ on the urgency on
the notification.

This allows to configure different notification parameters/scripts to different
urgency levels.
Fix URL matching regex

The URL matching regex as currently used by dust is too liberal, and will just
match about anything.

Some examples:

  "establishing connection..." => connection...
  "setting volume to [50%]" => 50%]

The following expression is an adapted copy of urxvt's URL matcher.
Merge branch 'match_msg_urgency' of git://github.com/wavexx/dunst into pull_requests
Merge branch 'fix_raise_hell' of git://github.com/wavexx/dunst into pull_requests
Merge branch 'fix_layout_alignment' of git://github.com/wavexx/dunst into pull_requests
Merge branch 'fix_memory_leak' of git://github.com/gs93/dunst into pull_requests
Merge branch 'feature-icon' of git://github.com/Gjum/dunst into pull_requests

Conflicts:
	dunstrc
	settings.c
Don't build dunstify on 'make' or 'make all'
use --std=gnu99 to get rid of compiler warnings