There was a bug that lead to the wrong path being returned by the function.

* Remove redundant return (code was never reached)
* Remove redundant type information
* Rename unused function parameters to "_"

* Get rid of open_darwin
	It just lead to code duplication for a simple one string change.
	Instead we query it during initialization
* Accept user provided arguments
	"open" on MacOS accepts things like -A to use a specific application
	Pass trough arguments the user provided in order to facilitate this
* Refactor the function to a struct
	This makes it more convenient for the caller and avoids signatures like
	lib.OpenFile(nil, u.String(), nil) which are fairly unreadable

Also update to the tcell v2 PaletteColor api, which should keep the chosen
theme of the user intact.

Note, that if $TRUECOLOR is defined and a truecolor given, aerc will now stop
clipping the value to one of the theme colors.
Generally this is desired behaviour though.

Prior to this commit, the composer was based on a map[string]string.
While this approach was very versatile, it lead to a constant encoding / decoding
of addresses and other headers.

This commit switches to a different model, where the composer is based on a header.
Commands which want to interact with it can simply set some defaults they would
like to have. Users can overwrite them however they like.

In order to get access to the functions generating / getting the msgid go-message
was upgraded.

We made a new type out of go-message/mail.Address without any real reason.
This suddenly made it necessary to convert from one to the other without actually
having any benefit whatsoever.
This commit gets rid of the additional type