Ctrl+L "Go to" should support -ve values for line and column numbers

Ctrl+L "Go To" can use negative line and column number convention. This will be helpful to say, jump to last line, 5th line from end etc. And this makes UI consistent for user. That is LINE and COLUMN can have -ve values and can be very handy.

Also it can use mousepad_util_set_real_line_offset() as currently it jumps by character not columns even though it asks user to enter COLUMN value.