[localthemedaemonclient] fix SVG prioritisation over inheritance

The whole model has been simplified: all file names (without path and extension)
are stored in one hash array across all inherited themes. Since extension is
ignored, the topmost theme to provide the graphic wins the race, no matter what
the file type is.
Merge pull request #14 from sledges/svg_fix

[localthemedaemonclient] fix SVG prioritisation over inheritance