Added class DBusProxy for easy DBus method invocation.
Improved support for locking the screen

It now works on most desktop environments, prefers D-Bus instead
of executing raw commands and the code is much better organized.

The code now supports two general methods of suspending: via DBus
(implemented in ScreenLockDBus.??) and via CommandLine
(implemented in ScreenLockCommandline.??).
Both classes inherit from a single interface: IScreenLockMethod.

Windows screen locking support is integrated in a separate commit.

The code was tested on Unity, Gnome3, KDE, Xfce, LXDE and i3 as of
Ubuntu 12.04.
Removed the block of previously commented code

Removed the block of previously commented code (it was commented
in the previous commit to make it more readable)
Integrated support of suspend on Win32 to the framework.
Trim includes in System.hh
Removed System::lockable
Added TRACE_MSG2 - printing a message from 2 strings
Better support for shutdown, introduced support for suspend/hibernate

Can use ConsoleKit, Logind and UPower, which should cover
vast majority of Linux systems in use (except for
very minimalistic distros).
Integrated support for shutdown on Windows
Exposed support for suspend/hibernate from System.hh

It is used in BreakWindow.
I tried to resize the buttons and the combobox so that their sizes
would match, but all attempts failed.
Added shutdown button to the RestBrake screen.

(Display of the button is configurable)
Fix configure.ac - bug with compiling under GTK2
Merge branch 'improve_lock_and_suspend' of https://github.com/matjon/workrave-1