removed unnecessary before selection code

with the document changed slot function now handling all lines changes,
it is no longer necessary to save information about the text cursor
selection before an operation

the Selection class was removed since it is no longer being used and
removed the reimplemented cut function since it is no longer necessary
to intercept it
updated new document changed code to work with undo/redo

because the cursor is not moved to the end of the change by an undo or
redo operation when the document contents change signal is received, the
current cursor cannot be used, so the document changed slot was modified
to obtain the document text cursor, change the position of the cursor to
the position of the start of the change, obtain info about this
position, moved to the end of the position (using the number of
characters added), and then get the info for this position (which the
cursor will be moved to afterward) - this change will also work with all
other kinds of changes

also renamed some local variables in the document changed function
removed the reimplemented undo and redo functions - not needed
removed checks for undo and redo key sequences
removed the undoAdded() slot - no need to keep track of line undos/redos
removed m_lineModCount and m_undoActive - not needed