New action mu4e-action-retag-message that reads tag changes from the user and
modifies the corresponding header (X-Keywords, X-Label, etc) on the message.
Now, mark commands with region on headers view,
target of the first email is reflected in the rest of email.
```
Date From Subject
--------------------------------------------
2012-12-05 Github ...
2012-12-05 Yahoo! ...
2012-12-05 Work ...
refiling to
Date From Subject
--------------------------------------------
r -> /github 2012-12-05 Github ...
r -> /github 2012-12-05 Yahoo! ...
r -> /github 2012-12-05 Work ...
```
In this patch, change target in region email.
```
Date From Subject
--------------------------------------------
r -> /github 2012-12-05 Github ...
r -> /yahoo 2012-12-05 Yahoo! ...
r -> /work 2012-12-05 Work ...
```
Autoload functions `mu4e' and `mu4e~compose-mail' and the definition of
the mail-user-agent `mu4e-user-agent'. The last to are required so
that `compose-mail' can be used without explicitly loading mu4e.
Also require `mu4e' in mu4e-compose.el (after providing mu4e-compose
to avoid a circular dependencies). This is required because the
autoload definitions only cause mu4e-compose but not mu4e to be loaded
when using `compose-mail', but if mu4e isn't loaded various handlers
are not loaded (and possibly there are other problems if mu4e is not
fully loaded).
Without this `mail-user-agent' cannot be set to `mu4e-user-agent'
through customize, as the custom type expects a function. Not sure
whether this function is actually ever used; if it is then returning
the symbol is probably the correct thing to do, as other such
functions suggest.
This allows `find-function' to find the definition. While the
definition doesn't contain much useful information jumping there
instead of the beginning of the file is still better because the macro
used to define them is defined right above.
Also remove the comments about wanting to define the commands in a
loop. One shouldn't do that; it would again make it impossible to
find the definition.
