titleeditor: Simplify the UI

Stripped the buggy UI so that the text field shows only the text,
without colors etc. The font and color buttons now take effect on the
entire text, not only on the selection.

Removed the bold and italic buttons as these are specified by the font
chooser button.

Fixes:
https://bugzilla.gnome.org/show_bug.cgi?id=726473
https://bugzilla.gnome.org/show_bug.cgi?id=708412