feat(widgets): let the quick timer start a custom duration

The quick-timer config screen now ends its list of presets with a
"Custom…" row that opens the timer keypad. Anything from 1 second to
59:59 is accepted; past that the keypad shows a range hint and OK stays
disabled, since the widget's readout is sized for MM:SS. A stored value
that isn't a preset marks the Custom row, with the value as its summary.

The keypad dialog is lifted out of the timer editor into a shared
TimerDurationDialog, so both use the same one; the editor's now also
shows the range hint the setup screen shows.

Closes #7

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

@@ -426,4 +426,7 @@
     <string name="widget_quick_timer_description">Starts a timer with one tap</string>
     <string name="widget_quick_timer_config_title">Quick timer</string>
     <string name="widget_quick_timer_start">Start %1$s</string>
+    <string name="widget_quick_timer_custom">Custom…</string>
+    <string name="widget_quick_timer_custom_title">Custom duration</string>
+    <string name="widget_quick_timer_range_hint">Between 1 second and 59:59</string>
 </resources>