theme: use 'labwc' directory

theme: use 'labwc' directory

as well as 'openbox-3', for example: /usr/share/themes/foo/labwc/themerc

Give 'labwc' higher precedence than 'openbox-3' if both exist.

For several reasons not all themes designed for labwc render well on
Openbox:

1. Labwc support some features like SVG/PNG icons and #rrggbbaa color
   definitions which Openbox does not.

2. Openbox generally defaults to black/white colors and will not render
   themes accurate without certain options specified in themerc. For
   example, in Openbox the following will just render as black:

       window.active.title.bg.color: #589bda

   ...because Openbox defaults the texture to "Gradient Vertical" and
   therefore either needs:

      window.active.title.bg.colorTo: #3c7cb7

   ...or remove 'Gradient' from the bg definition and just use something
   like:

      window.active.title.bg: Flat Border

Whilst none of this is a problem when using labwc, it causes problems for
setups with both Openbox and labwc because themes designed for labwc only
will now show up in obconf, lxappearance, etc causing confusion for users.