As a consequence, move to use an explicit handshake() method rather than
calling virtuals from the constructor. This seemed to not bother
AClientConnection, but LibGUI crashes (rightfully) because of it.
Taskbar now simply asks the WindowServer to popup a window menu when right
clicking on a taskbar button.
This patch also implements the "close" menu item, and furthermore makes the
window menu show up when you left-click a window's titlebar icon. :^)
