Update FirstRun.pm to let `kdesrc-run` call executables in subdirectories.

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/picmi,
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. It's consumed 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. picmi instead of picmi.
  Edit: Separating the quoted command from the optional arguments does the same, but simpler.

This MR will allow to call a project from it's respective subdirectory, such as:
kdescr-run playground/utils/basket
but it will maintain its functionality if the project is conventionally located in kde/src/