Gtk::AppChooser*: Improve documentation

* gtk/src/appchooser.hg:
* gtk/src/appchooserbutton.hg:
* gtk/src/appchooserdialog.hg:
* gtk/src/appchooserwidget.hg: Copy class documentation from gtk+ that was
added to gtk+ after the classes were created in gtkmm. Add links to
giommContentType in glibmm. See gtk+ bug #650012. These links will be broken
until a new version of glibmm (after 2.38.0) is released.
Docs: Add missing widget images

* gtk/src/colorchooserdialog.hg: Change image file name.
* docs/reference/images/widgets/appchooserbutton1.png:
* docs/reference/images/widgets/appchooserdialog1.png:
* docs/reference/images/widgets/colorchooserdialog1.png:
* docs/reference/images/widgets/infobar1.png: New files, copied from gtk+.