'marking' & selective queue management

Tracks may be marked and unmarked with win-mark! and related functions.
win-add! and win-remove! are modified to operate on all marked items
(the selected item counts as marked).  Color options are added to
visually distinguish marked/unmarked items.
echo! function

Function to display text on the command line.  Uses the format function
with "~a", so that arbitrary scheme expressions can be echoed.
win-move-tracks! & queue-* functions

The win-move-tracks! function moves all marked tracks (excluding the
selected track) to the position "under the cursor" (before the selected
track).

The queue-* functions are wrappers from scmus-* functions which affect
the queue (delete, move).
Unicode support in editable text

It is now possible to enter unicode characters into the command line.
Options view/window

The options view simply lists the scheme values for all options, similar
to the status view.
option-string function

The option-string function takes an option and returns a string
representation.  This is used for displaying format strings in the
options view.