Show a warning when try to open a big file

[Yuri Malheiros]

TextFlow supports drag and drop to open a file, then someone could try to open a big file, for example a image or a pdf, with TextFlow. A file like these has thousands or millions of lines in text format and Gtksourceview could crash trying to display the text.

Sometimes a user really wants to open a big text file with thousand of lines, this could be a little slow, but gtksourceview can open. A warning dialog will give to user the possibility to choose if he or she really wants to open a big file.

[Waldecir Loureiro dos Santos Filho (waldecir)]

i ahve attache the patch for this feature.