xkcd_2.7.8

• Add settings option to select external comic cache when there is an SD card
• Add settings option to show comic title text on preview page
• Improve fullscreen comic page UI when navigation bar is used
• Localize comic release date
• Enable comic list filter by default even no thumb-up or favourite is made
• Enable xkcd notification for devices without Google Service
• Fix comic image needs to be re-downloaded even it is presented in the notification
• Remove unnecessary network cache