When a project source directory is located in a subdirectory of `kde/src/`,
such as `kde/src/playground/utils/basket`, or `kde/src/kde/kdegames/picma`,
the `kdesrc-run` function will either not be able to locate the `prefix.sh`
in the respective path or it will fail to find that path in `kde/usr/bin/...`

This commit extends the approach of arguments expansion `$@`.
* `${1##*/}` will remove any optional prefix directory:
  `playground/utils/basket` will be shortened to `basket`.
* `${@:2:$#}` will expand the argument list, but the first which is dealt with above
* `${2:+ }` inserts a space only if there is a second argument,
  otherwise the `"` quoting would always append a literal space to the command,
  e.g. `picma ` instead of `picma`.