refresh cache on startup, gtk 3.8 fixes, podcast fixes, ...
- option to refresh episode cache on first visit of a programme type;
- workaround desktop item not shown in (kde) desktop menu (.desktop file name too long);
- temp fix set row height in compact tree view in gtk 3.8;
- fix exception when leaving prefs dialog (FileChooserButton can return None in gtk 3.8);
- fix podcast play url, play title and image (don't display large (podcast) images in the tree view tooltip and properties window) + (truncate title in the properties window header section to avoid scroll bars);
- fix 'episode without description' in tree view;
- fix download log (only log get_iplayer.get() in a separate *-cmd.log file);
- add silent option when command returns error code;
- minor text updates