move Reflow option to More options dialog
add pre-rendering in pdf/djvu readers
"HintPage" tasks are scheduled into the execution stack of UIManager
after each page drawing. These tasks should be checked immediately after
screen is refreshed so tasks are double-checked in the main loop.
Merge pull request #685 from chrox/option-tweaks

move "Reflow option" to "More settings" dialog
Merge pull request #686 from chrox/prerendering

add pre-rendering in pdf/djvu readers
use ReaderRolling.view_mode instead of self.ui.document.view_mode

view_mode is removed from self.ui.document