feat(worldclock): add an option to display time with seconds

One switch in Settings > Clock adds seconds to the home readout and
every city row, in the same 12/24-hour and locale form as before, and
gives the analog face its second hand. Off by default.

The tab now ticks on the wall-clock boundary through a new
Ticker.alignedTicks: once a minute with seconds off, once a second only
with them on, replacing the free-running 200 ms tick. The setting is in
backups as world_clock_show_seconds; older files import with it off.

Fixes #23

app/src/main/res/values/strings.xml

@@ -301,9 +301,11 @@
     <string name="settings_timers_sound_subtitle">Sound for new timers</string>
     <string name="settings_timer_sound_hint">New timers ring with this sound. Each timer can still change it in its editor.</string>
     <string name="settings_section_clock">Clock</string>
-    <string name="settings_clock_subtitle">Home time zone</string>
+    <string name="settings_clock_subtitle">Home time zone and seconds</string>
     <string name="settings_clock_hint">World clocks show how far each city is from your home time zone.</string>
     <string name="settings_home_zone">Home time zone</string>
+    <string name="settings_world_clock_seconds">Display time with seconds</string>
+    <string name="settings_world_clock_seconds_hint">The world clock shows seconds and a second hand. Uses a little more battery.</string>
     <string name="settings_self_check">Why might my alarm not ring?</string>
     <string name="settings_self_check_title">Why might my alarm not ring?</string>
     <string name="settings_self_check_summary">Check exact-alarm, notification and battery settings</string>