Add new feature: ClearPlaylistOnFileOpen configuration

Add new configuration option "Clear playlist on file open". When activated, Elisa clears the playlist before files are loaded.

Elisa wasn't able to just simply play a file when opening it from Dolphin or dragging the file on the window. It would add the files to the playlist but not play them. I added a configuration option which clears the playlist before any files are added to the playlist, in combination with the "play on startup" config, it is simple to just play the files by double click/drag & drop. For me, this was a killer feature.

Please review the code if it is OK :) Thank you