Move interaction functions to a separate file

This is a first step to clean up several compilation units and separate
the front end from back-end operations. All functions that require user
interaction are moved to a new compilation unit "interaction.c". Also,
following things are adjusted to the new layout:

* Make day_item_get_*() and a few other functions public, so that it can
  be accessed from the new compilation unit.

* Use apoint_hilt(), todo_hilt(), etc. instead of directly accessing
  static variables.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

po/POTFILES.in

@@ -11,6 +11,7 @@ src/event.c
 src/getstring.c
 src/help.c
 src/ical.c
+src/interaction.c
 src/io.c
 src/keys.c
 src/llist.c