Export online help as text files

Move the texts of each online help screen to a separate text file. The
set of files is supposed to replace the online help screens later.

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

doc/edit.txt

@@ -0,0 +1,18 @@
+Edit Item
+=========
+
+Edit the item which is currently selected.
+Depending on the item type (appointment, event, or todo), and if it is
+repeated or not, you will be asked to choose one of the item properties
+to modify. An item property is one of the following: the start time, the
+end time, the description, or the item repetition.
+Once you have chosen the property you want to modify, you will be shown
+its actual value, and you will be able to change it as you like.
+
+Notes:
+     o if you choose to edit the item repetition properties, you will
+       be asked to re-enter all of the repetition characteristics
+       (repetition type, frequence, and ending date). Moreover, the
+       previous data concerning the deleted occurences will be lost.
+     o do not forget to save the calendar data to retrieve the
+       modified properties next time you launch Calcurse.