mate-keyboard-properties: catch (and print) any errors during UI load

the code is taken from mate-keybinding-properties, so it may or
may not be optimal, feel free to correct it if you wish.
mate-keyboard-properties: removed property that made UI load fail (GTK+3)

same as https://github.com/mate-desktop/mate-control-center/commit/40146341b70b93c0ce142580b0860b01f7072364
mate-keyboard-properties: fix indentation
set update_policy in the code for GTK+2 build