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>

src/Makefile.am

@@ -21,6 +21,7 @@ calcurse_SOURCES = \
 	getstring.c \
 	help.c \
 	ical.c \
+	interaction.c \
 	io.c \
 	keys.c \
 	llist.c \