Start to make it easier to compile the core in isolation

Start to make it easier to compile the core in isolation

This is for work on making the files scannable by GObject-Introspection
but is still useful otherwise (even fixes a FIXME in the comments). I
made this by using a simple GNU Make file and trying to compile the
sources each on their own without all the build system infrastructure.

* Add keybindingsprivate.h file to hold private GeanyKeyGroup structure
and remove it from the GEANY_PRIVATE guard in keybindings.h.
* Move private members that were guarded by GEANY_PRIVATE from
GeanyFiletypes to GeanyFiletypesPrivate and remove guarded build.h
include.
* Move private members that were guarded by GEANY_PRIVATE from
GeanyProject to GeanyProjectPrivate.