Added a simple startup script for SmallWM.

This can be used as a script for an entry into /usr/share/xsessions, for example.
Fixed two bugs related to clients.

SmallWM would run a clients ClassActions every time it was mapped and clobber the previous
state of the window. Client windows are now checked for existence before they are created.
Removed all the old C files.

The C++ port is now considered to be on par with
the original C version. If you want to use the old
C version, then you can checkout the new `legacy`
branch.