Wallpaper: hide color or blur filling options for full filling mode

Summary:
Related to patch https://phabricator.kde.org/D7047 . Pointed out by @ngraham

Make a new option called "Edge filling". Users can choose either "Blur image" or "Solid color". They are only visible when the image filling mode is "Center" or "Scale and keep ratio".

When it is "Center":

When it is "Tile":

Reviewers: ngraham, jensreuterberg

Reviewed By: ngraham, jensreuterberg

Subscribers: jensreuterberg, broulik, ngraham, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8441