Adds a screensaver mode which is just a line of text at the top of the screen. Purpose is to be minimally obstructive while still displaying something to indicate the device is asleep.
* Fix the "Enable debug logging" checkbox so that it properly disables "Enable verbose debug logging" when it gets disabled
* Avoid asking ImageWidget for alpha-blending when it's not useful
* Make ImageWidget's alpha-blending code-path double-check that alpha-blending actually is needed, and avoid it if possible
* In the same vein, only do alpha-blending in textboxwidget when absolutely necessary
* Prefer color constants over the gray() method, ensuring that they're part of the eInk palette
Depends on https://github.com/koreader/koreader-base/pull/853Fix#4774
TouchMenu: added options to menu items with the following defaults:
keep_menu_open = false
hold_keep_menu_open = true
So, default for Tap callback is to close menu, and for Hold callback
to keep menu open.
In both cases, provide the TouchMenu instance as the 1st argument to
the callback functions (instead of a refresh_menu_func I added in #3941)
so the callback can do more things, like closing, refreshing,
changing menu items text and re-ordering...
ReaderZooming: show symbol for default (like it was done for
ReaderFont, ReaderHyphenation...)
TextEditor plugin: update the previously opened files list in real
time, so the menu can be kept open and used as the TextEditor main
interface.
SSH plugin: keep menu open and update the Start/Stop state in real time
ReadTimer plugin: tried to do what feels right (but I don't use it)
Also remove forgotten cp in the move/paste file code
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
