Add keepalive functionality for C programs that utilize glib mainloop

Originally keepalive functionality was written with only Qt and
QML applications in mind. This meant that system daemons and
middleware components that need to deal with waking up from suspend
and/or staying out of suspend had to use lower level services directly.

Implement feature-wise similar interface for use from glib based
programs as what is available for Qt/QML applications.

Interfaces provided:
* keepalive-displaykeepalive.h disable based display dimming/blanking
* keepalive-cpukeepalive.h disable autosuspend policy
* keepalive-heartbeat.h wake up from suspend via iphb service
* keepalive-backgroundactivity.h periodic wakeups with suspend blocking
* keepalive-timeout.h suspend aware g_timeout_add like functionality

Also includes simple example applications.