Problem without batch lazy loading

[linonetwo]

In calendar plugin, if trigger lazy loading for a week of events, they will load one by one, cause lots of rerender. And the loading time is unpredictable, so a smaller debounce time in refresh method will sometimes not work, larger debounce time will make lagging experience. And many of them don't even have text.

There are two things I want to make change:

1. only add _is_skinny if a tiddler have text field, so we only need to load those tiddlers really have text.
2. allow lazy load to batch, by collect requirements in this tick, and use settimeout 0 to do it in next tick, like wiki batch operation does.

This is not only for calendar plugin, but also for gallery plugins, they load images one by one, for hundreds of images, which might take 1 min to load, and cause lots of rerender.