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 *ComboDn_12_xpm[] = {
/* width height ncolors chars_per_pixel */
"12 12 2 1",
/* colors */
"a c black",
"b c none",
/* pixels */
"bbbbbbbbbbbb",
"bbbbbbbbbbbb",
"bbbbaaaabbbb",
"bbbbaaaabbbb",
"bbbbaaaabbbb",
"bbaaaaaaaabb",
"bbbaaaaaabbb",
"bbbbaaaabbbb",
"bbbbbaabbbbb",
"bbbbbbbbbbbb",
"bbaaaaaaaabb",
"bbbbbbbbbbbb",
};