The dashed line GC is set before user options are parsed, so
only newly created DeskStyles will have the dashed_gc set to
a solid line. This means that when showing multiple desks, no
desks will have a solid line, or when showing the current desk,
the initial desk will not have a solid line, but other desks
will.

The label_gc uses the same fg color as dashed_gc, and can be used
to create solid lines. So instead of relying on dashed_gc to know
if solid lines should be used or not, use either label_gc or
dashed_gc to draw the separators depending on if the option
SolidSeparators is used or not.

Fixes #1138