v0.5.0

*new @frow / @fcol functions.
*Added :file special command.
*:cellcolor command now can take a range as an argument.
*Added gg command and HOME, END, PAGEUP and PAGEDOWN keys handling in help screen.
*CSV load/import time improvements (noticeable in large files).
*Fix error when ordering a range that had empty cells.
*Fix in csv import (isnumeric function).
*Fix when circular reference occurs:
 Before it put @ERR in expression.
 Now it preserves the var that makes the circular reference in the formula,
 but keeps the "ERROR" label in the ent.
 ERR errors propagate to other ents.
*Changes when an ent referenced by other ents is removed:
 Before, @ERR was set whereever the reference to a removed ent exists.
 Now, if an ent is removed because of:
    1. a dr dc sh sk command,
    is_deleted flag is set on those ents.
    eval return 0 for those cases, and CELLREF is put in e->op.

    2. 'x' command, flag is_deleted is not set.
    eval return 0 for those cases as well.

 REF and ERR errors propagate to other ENDS.
 Another change: Now when deleting a cell, if they are referenced by others, vertex is not removed.
 (is_deleted ents shall always go to freeents to be reused..)
*Added check for not to copy duplicate ents in copy_to_undostruct.
*Removed split function.
*Removed unnecesary lookat in screen.c that made unneeded mallocs.
*With "debug" configuration parameter set, now SC-IM quits after SIGINT.

*Fix bug that prevented cells with "ERROR" get saved to file and loaded later on.
*Fix BackSpace issue with some GNU/Linux distros.
*Fix bug that create duplicates in commandline history, if the command recently typed was the latest used as well.
*Fix bug that avoid processing $HOME/.scimrc after loading a file via commandline.
*Fix bug with 'Pc' and 'c' commands of normal mode, not doing autocalc after paste.
*Fix in possible buffer overflow in main.c.
*Some fixes in xml files import.
*Various fixes to avoid warnings during build.