Wayland.cmake: Fix typo

Correct typo.
Wayland.cmake: Generated code depends on client header

This way we are sure to generate client headers too.
libgreenisland: Namespace

Put the code into a namespace to avoid conflicts.
Require Qt 5.2 or better
Remove QtWidgets dependency

We no longer need it.
QtCompositor is now a hard dependency
Exclude the old executable from the build

However we should save some utility code so this stays here until
we move it into the library.
README.md: Update instructions

We no longer provide an executable with Green Island since it ships
a library and a QML plugin now.
Update also the dependencies list.
libgreenisland: Utilities functions

Move utilities code from the executable to the library.
Remove greenisland executable