When no manually added specific 'poweroff' or 'reboot' screensaver
settings, use the suspend/sleep settings set via menu with an added
overlay message in top right corner.
Generalized some settings: "White background behind message and images"
and "Stretch covers and images to fit screen", and move "Exclude this
book's cover from screensaver" up a level (and remove this menu item
added by ReaderMenu when closing document).
Don't use memory cache with rendered screensaver images.
ImageWidget: fix wrong behaviour (stretch) when 'file', 'width',
'height' and 'scale_factor' provided: now, keep aspect ratio.
ImageWidget: increase file image mem cache (mostly used for icons)
from 2 to 5MB to be ready for devices with very high DPI.
Use "message" as the default for screensaver (instead of "disable") (https://github.com/koreader/koreader/pull/3535#issuecomment-353337143)
On Kobo
Fix: display message on poweroff or reboot (https://github.com/koreader/koreader/pull/3535#issuecomment-352355865)
Allows for overwritting these default messages with the classic screensaver settings, by just prepending "poweroff_" or "reboot_" to them. (https://github.com/koreader/koreader/pull/3535#issuecomment-352498497)
So one can manually add to its settings.reader.lua:
```
"poweroff_screensaver_type" = "random_image" or "message", "cover", "bookstatus", "readingprogress", "disable",
"poweroff_screensaver_dir" = "path to same as screensaver_dir or alternate dir for poweroff image(s)",
"poweroff_screensaver_message" = "message to use instead of Powered off",
```
```
"reboot_screensaver_type" = "random_image" or "message", "cover", "bookstatus", "readingprogress", "disable",
"reboot_screensaver_dir" = "path to same as screensaver_dir or alternate dir for reboot image(s)",
"reboot_screensaver_message" = "message to use instead of Rebooting...",
```
* Prevent open screensaver more than one in the same time
* Add option to enable/disable background in message screensaver
* fallback to random images if this book cover is excluded feature + fix Disabled (leave screen/page as it is)
* Autostretch disable by default, more fallback options, leave screen as it is
