Allow Input of local includes from local layout files

Allow Input of local includes from local layout files

When including files, LyX always searches the user and the system
directory, in that order. This means that when using local layout
files broken down into multiple includes, the various includes should
be specified with a path relative to the user layouts directory
(typically ~/.lyx/layouts), making this very impractical.
This commit allows including local files by specifying their path
as explicitly relative to the main layout file position, i.e., by
specifying their path with either "./" or "../". If the main layout
is not loaded from a local file, the usual search order is used,
even if the path are explicitly relative. So, for system layouts,
both "Input ./name.inc" and "Input name.inc" are equivalent.