Use configure_file instead of file(COPY ...).

This way, the desktop files are properly updated in the build dir
whenever they are changed in the source directory.
Add Endpoint::isRemoteClient which can be used in the future.

I thought I might need it now but found a different way after all.
Still, Volker agrees that this will be useful sooner rather than
later.
Dont enable plugins on the client side which do not support remoting.

This is achieved by a new remotingSupported API in the ToolFactory,
which is returning false for StandardToolFctory (1) and true for
StandardToolFactory2 by default. Plugins can overload this and
return false and should then set X-GammaRay-Remote=false in their
.desktop file.