kdeplasma-addons: use URL save ID appropriate for the POTD wallpaper plugin

kdeplasma-addons: use URL save ID appropriate for the POTD wallpaper plugin

there is a frame applet and the dialogs for the POTD wallpaper and frame
applet should not share the same save URL (questionably).

perhaps there should be feature to save the URL based on mime type group,
for example save "text/plain" and "text/x-patch" as the same URL for all
applications by default unless a custom save location has been specified
programatically

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>