This allows to recur through other timers. Previously timer could only be self-recurring.
Previous implementation allowed recurrence in the form (A, B, C denote alarms):
A -> A -> A ...
Current implementation allows any of the following:
A -> A -> A ... A -> B -> A -> B ... A -> C -> B -> A -> C -> B ... C -> A -> B -> A -> B ...
This replaces and extends 'Recurring alarm' option into combo box that allows selection of subsequent alarm to run on timeout. Any alarm can be selected or none of them.
Reading timer plugin rc file is backward compatible, i.e. it respects is_recurring setting. But on save this setting will be replaced.