Merge branch '3598_wcast_qual'

Merge branch '3598_wcast_qual'

* 3598_wcast_qual: (27 commits)
  src/setup.c: cleanup -Wcast-qual warning.
  src/clipboard.c: cleanup Wcast-qual warning.
  src/viewer/search.c: cleanup some -Wcast-qual warning.
  src/vfs/extfs.c: cleanup -Wcast-qual warnings.
  src/filemanager/find.c: cleanup -Wcast-qual warning.
  src/filemanager/file.c: cleanup -Wcast-qual warning.
  src/filemanager/file.c: cleanup -Wcast-qual warning.
  src/filemanager/panel.c: cleanup -Wcast-qual warning.
  src/editor/syntax.c: cleanup -Wcast-qual warnings.
  src/editor/editcmd.c: cleanup some -Wcast-qual warnings.
  src/editor/choosesyntax.c: cleanup -Wcast-qual warning.
  src/diffviewer/ydiff.c: cleanup -Wcast-qual warning.
  (dview_get_utf): apply const qualifier to str argument.
  lib/charsets.[ch]: clarify usage of const qualifier.
  lib/widget/input_complete.c: cleanup -Wcast-qual warning.
  lib/widget/input.c: cleanup -Wcast-qual warning.
  lib/widget/dialog.c: cleanup -Wcast-qual warning.
  lib/widget/buttonbar.c: cleanup -Wcast-qual warning.
  lib/vfs/path.c: cleanup some -Wcast-qual warning.
  lib/tty/tty-ncurses.c: cleanup -Wcast-qual warning.
  ...