Merge lp://staging/~teemperor/pantheon-greeter/wallpaper-settings-from-file into lp://staging/~elementary-pantheon/pantheon-greeter/trunk

LightDM-API doesn't supply things like scaling-method and primary/secondary-color via it's API and there is no way to read a dconf-db on in a folder where we don't have the access rights. Also, the LightDM Api seems to no longer recognize our way of determining the wallpapers (?).

So we read from now on the necessary information for the wallpaper from a file from the disk.
The file is named greeter-settings and is just a set of UTF-8-encoded key-value pars.
It is located directory in the home-folder of each user as we can read that on elementary OS due to 755 rights.

For other distributions like arch (which use 700-rights by default) we need to make a workaround with an external directory in case the user wants it background displayed without making his home 755 or 711.

This merge adds the functionality to the greeter that we read the file from the disk and display the wallpaper itself. Things like scaling and color are ignored for now, if i would add this too to the patch we would end up with a humongous match that nobody wants to review (like last time when i started dumping patches on lp).

Things that should be tested:

* It can read the wallpaper from the disk and displays it in any form (scaling is as said not the target of this patch).

* It doesn't crash on a malformed greeter-settings file

You can use this program [1] to generate a greeter-settings file for now. It's up to decision on what way this file will be written (gala-plugin, cerbere-service, directly from the shell-plug).

Cheers

[1] http://pastebin.com/pUD257Xw