fix desktop file install paths

[popglitchx]

couple of small build fixes:

• icon and metainfo were being dumped into share/applications alongside the .desktop file. icon wouldn't show up in launchers since the filename didn't match the Icon= key either. split them into their proper directories.
• mlspp shim config from a prior run would get cached in MLSPP_DIR, making cmake skip the subproject on subsequent configures and breaking libdave's link step. unset the stale cache vars before find_package and moved the alias creation outside the if block so targets are always available.