Use KDE file dialogs

The one element that would improve the user experience on KDE the most is the usage of the native file open/save dialogs.

This is not only about the look, but also about functionality, e.g. favorite folders.

Implementation is straightforward by using the `kdialog` CLI utility.

One project that does this already is Chromium, using the xdg-utils.

Starting point reference: https://code.google.com/p/chromium/codesearch#chrome/src/third_party/xdg-utils/scripts/xdg-file-dialog