ReaderStatus: update status modification time

[hius07]

In the "End of the book" action.
Closes #11564 (No idea how we got a book without modification time but the time stamp must be updated in any case)

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This change is 

[Frenzie]

Every time? Or put another way, what's this used for exactly? :-)

[hius07]

We change the timestamp when changing status.

At opening a book

koreader/frontend/apps/reader/readerui.lua

Lines 481 to 482 in 424fa9b

	summary.status = "reading"
	summary.modified = os.date("%Y-%m-%d", os.time())

In Book status window

koreader/frontend/ui/widget/bookstatuswidget.lua

Lines 231 to 232 in 424fa9b

	self.summary.status = option_name[option_value]
	self.summary.modified = os.date("%Y-%m-%d", os.time())

In file popup dialog

koreader/frontend/apps/filemanager/filemanagerutil.lua

Lines 120 to 121 in 424fa9b

	summary.status = status
	summary.modified = os.date("%Y-%m-%d", os.time())

So let's do it in the same way at the End of the book

koreader/frontend/apps/reader/modules/readerstatus.lua

Lines 221 to 222 in 3c839a6

	self.summary.status = (not mark_read and self.summary.status == "complete") and "reading" or "complete"
	self.summary.modified = os.date("%Y-%m-%d", os.time())