Fix scrolling right in the TUI

This commit fixes two issues in scrolling right in the TUI:

#1 - Scrolling right with the arrow keys, the first keypress doesn't
do anything.  The problem is that copy_source_line() checks if
(column < first_col), and because of the ++column directly before, it
basically starts with 1 instead of 0.

#2 - Scrolling right handles TABS and escaped characters as single
characters, which just looks weird.  The problem is that there's a
spot that misses handling TABS.

gdb/ChangeLog:
2019-03-18  Hannes Domani  <ssbssa@yahoo.de>

	* tui/tui-source.c (copy_source_line): Fix handling of 'column'.
	Handle tabs.