fixed issue #18

Fixed bug #18. This bug was located inside `QucsApp::slotMenuDelProject`. This method gets project directory name from the `QFileDialog`. It assumed that this directory name was ended with `QDir::separator()` symbol. Then this symbol was removed from the of the string. In fact, `QFileDialog` returns directory name without trailing `QDir::separator()`. This fact leads to that bug that project name is determined wrong, when last 4 symbols (`_prj`) are truncated. Now checking of trailing `QDir::separator` is added. Now Qucs removes the last symbol of project directory path only if `QDir::separartor()` is present. And project always is removes correctly now. Also slash (`/`) symbol is replaced by platform independent  `QDir::separartor()`.