Before this patch a `t edit ...` will remove todo items selected by
`...` and then do equivalent of `t add` with input from result of
editing. This works, but has the property that whenever a todo item is
edited, it is always moved till the end of todo.txt file.

This property creates noise and makes synchronization harder to implement for
some of us who uses VCS and different machines to maintain todo.txt .

-> Teach edit to preserve todo items in their places. Adjust existing
tests and add a couple more exercising cases where either new todo
items are added in the editor, or vice versa, removed in the editor.

/cc @mruwek

Version 4.2.5 that is installed on Travis prints false positives causing
checks to fail.

Make sure that version 4.3.x or higher gets installed.

topydo may still work but will not be tested against this Python version

When you enter a todo item: "Call office at 09:00", the 09:00 would be
seen as a tag-value pair. Because of this, the timestamp would be
removed from the todo text: "Call office at ".

Discussed in issue #211.

Also, display the last changelog at the bottom of the homepage

Related to issue #194.

Suggest to use pip3 over pip

`topydo revert ls` will now show all saved backup points with position
number (starting with 1), date and time of relevant action, and its
label.

`topydo revert 2` will revert to a point **before** the backup point
marked as 2 in `topydo revert ls` (i.e. it will go 2 changes backwards
or phrasing more naturally: revert last 2 changes).

Warning: changes between between current state and desired backup point
are also deleted to avoid false assumption that our backups can be
treated as some sort of atomic "patches".

When user disabled archive feature after he made some backups, don't try
to restore archive during `topydo revert`, as there is no file to write.

self.todolist may not be present; it could be None. This is the case
when the list formatter is instantiated from a TodoWidget in the column
UI, which passes None for the todo list.

The overrides are necessary to make the multicommands/widgets behave
well.

'test' is not really a library that has to be imported

Among others, moved methods by turning them into (inline) functions or
static methods.