utils: Introduce RamFile class for memfd

This class can be used to create an anonymous file, for instance
to pass data between compositor and clients, through means of a
file descriptor, as is done in various Wayland protocols, notably
the keymap exchange.

It also implements sealing the file, so that it can be shared
between multiple clients without them being able to modify it.

If supported, memfd_create is used, otherwise a `QTemporaryFile`
is used.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>