Print version number and build time on startup

Ref #41
Revert "config: refactor config getter functions"

This reverts commit a34090699a62e5c7efe43ba0b1a31079215f5246.

Will try to find a better way to do this later.
widgets: update debug code to work with old config funcs

Ref #42
Add override_redirect flag to wkline's window

wkline now works in dwm (and should work in monsterwm)

Closes #45
waf: add build time to wkline.c defines

This adds the defines only to wkline.c to avoid rebuilding the entire
project.

Ref #41
config: simplify fetching widget configuration

Closes #41
util: add GDK helper library

Added function to fetch supported EWMH properties.

Based on example code from
http://opensourcejavaphp.net/csharp/gtksharp/windowmanager.c.html
main: only set override redirect if EWMH is unsupported
main: use correct atom name (_NET_WM_STRUT)
widgets: don't bother supporting an enabled property yet

Ref #41
main: add web view init function

This function disables all third-party plugins and sets various webview
settings before returning the webview.
main: handle both strut and strut_partial properties
Use correct indexes on strut partial array
config: add convenience macros to fetch config values

Ref #44
config: rename widget JSON config pointer to json_config
config: add macros for overriding default config values
widgets/external_ip: implement config with default values
widgets: implement new config system for remaining widgets

Ref #44
config: remove config overrides from config.json
widgets: add datetime widget