This adds an extra step to the lookup of SVG images created with the
Svg() global function: after looking in images/ in the plasmoid and then
in the desktop theme, it looks in theme/$DESKTOP_THEME_NAME and then
theme/ in the plasmoid.
This allows plasmoid authors to add images that they want to allow theme
authors to override.
Forward-port of review request 109857.
All cpp code moves into the src/ subdirectory, as the Frameworks policy
suggests.
Directory structure should now be in line with other, future frameworks.