compositor/main: Extract split/retrieve args passed shell client

The supplied path for executing the shell client could contain potential
arguments and not only the binary itself. Specifying the platform either
by using an argument (-platform wayland) or using an environmental
variable (QT_QPA_PLATFORM) with clients written in Qt is for instance
an example on why this might be useful to have in.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>