optionally make folderview's background transparent

[aseigo at bddf.ca]

> since either none of our review boards work or I do am not able to make
> those work, I'll just attach the patch here.

i'm hoping we can get http://reviewboard.vidsolbach.de/dashboard/ working;
it's on the same server as plasma.kde.org so would be convenient for me =)

> It is self-explanatory (if you read the subject). =)

doing this in each and every applet (which is where these kinds of patches
lead us) is really wrong.

there are two things that i can see happening here that are, imho, more
palatable solutions:

* provide a generic option in Applet itself for this (so, no per-applet code)

* improve the artwork for the default background such that it doesn't
annoy people as much

honestly, this is really an issue with the artwork we have currently.