Add infrastructure to remember last selected folders
Save last folders to ~/.4deckradio
Implement config file IO

Simple methods to read and write lines to ~/.4deckradio
Fix potential memory leak in GError handling

Instead of setting the pointer to NULL, call g_clear_error() to free
the associated message char array.
Colour timelabels according to percentages

Use the following map:

    0 ..   50% elapsed: green
  50 ..   75% elapsed: yellow
  75 .. 100% elapsed: red