Add TextView get text attribs functionality

[frang75]

In https://nappgui.com/en/howto/guihello.html#h6 we can apply the current text attributes to all/selected text in control, using:
textview_apply_all()
textview_apply_sel()

In order to implement a full-featured text editor, we need to access the text attributes (font, size, color, bold, ...) of each current character in the text view.

Related to: #147