Issue #130: Fixed support for indenting by tab
Allow indent a block of code by selecting a range of lines
and pressing 'tab'. The behavior implemented is the same as
the one in Sublime Text editor since I found it intuitive enough.
/* XPM */
static char *ScrollDn_disabled.w95.9_xpm[] = {
/* width height ncolors chars_per_pixel */
"9 9 3 1",
/* colors */
"a c white",
"b c #808080",
"c c none",
/* pixels */
"ccccccccc",
"ccccccccc",
"ccccccccc",
"cbbbbbbbc",
"ccbbbbbac",
"cccbbbacc",
"ccccbaccc",
"ccccacccc",
"ccccccccc",
};