datetime: Implement listbox based overview

... and move existing date/time and timezone settings to separate subdialogs.

https://bugzilla.gnome.org/show_bug.cgi?id=694985
datetime: Clean up error handling

Avoid using two GError variables in a single function to reduce
programmer confusion. Instead, only use one and clear it with
g_clear_error() when we need to reuse it.

Pointed out by Rui Matos.

https://bugzilla.gnome.org/show_bug.cgi?id=694985
datetime: Show the timezone map in the original, unscaled size

https://bugzilla.gnome.org/show_bug.cgi?id=694985
datetime: Move the 24h/12h selection to the main overview

... and use a combobox for choosing between the options.

https://bugzilla.gnome.org/show_bug.cgi?id=694985
datetime: Implement new design for the time subdialog

https://bugzilla.gnome.org/show_bug.cgi?id=694985
datetime: Shuffle initialization code around

Move subdialog setup code out of the main init function and to
respective setup_*_dialog() functions.

https://bugzilla.gnome.org/show_bug.cgi?id=694985
datetime: Toggle the NTP switch when clicking on the row

Pointed out by Allan Day on IRC.

https://bugzilla.gnome.org/show_bug.cgi?id=694985
power: Avoid warning with no keyboard backlight

If there's no keyboard backlight interface in gnome-settings-daemon,
hide the slider as well, rather than throw a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=706040
power: Avoid warning with no screen backlight

Similar patch to f7c5225288b03c7a6f8139a1c02b840c7eddda8d
but for the screen backlight.