* src/undostack.{cc,h}: New files, implementing the core of the
new undo system.  Basically it's a simple undo stack using virtual
methods for the implementation of undo actions.  But the undo stack
is itself an undo action, which allows for nested stacks.  This
functionality is necessary since the undo works globally on all
files.

* src/filebufferundo.{cc,h}: New files, defining the implementation
of the FileBuffer undo actions.

* src/fileshared.{cc,h}: Move parts of filebuffer.{cc,h} into these
new files, in order to avoid circular dependencies and to reduce the
length of single source files.

* src/filetreeprivate.{cc,h}: Move the anonymous namespace parts
and the nested classes of the FileTree implementation into separate
files.  filetreeprivate.h is included by filetree.cc only.

* src/Makefile.am (regexxer_SOURCES): Add new source files.

* src/filebuffer.{cc,h}: Heavily modify and enhance the code in
order to support the new undo framework.

* src/filetree.{cc,h}: ditto.

* src/controller.{cc,h}: Add undo ControlItem and show menu/toolbar
items for it.

* src/mainwindow.{cc,h}: Add code to maintain the undo stack and
connect the GUI bits.