Caching mechanic

[LancerComet]

FileMeta

FileMeta is the abstraction of a single file. It should make file loading faster.

• [x] Can analyze all files under a bookshelf folder. There should be a button to do this in the folder page.
• [x] Start automatically when a new folder is being added
• [x] Get the pixel size of every single image in archive and store them
• [x] Use FileMeta when loading a file
• [x] Create the FileMeta when a file has been loaded into the reader
• [x] Files in small size or have few images should not be cached
• [x] There should be one or more indicators to tell users that files are being analyzed
• [x] Make a helping dialog for the first time usage

Thumbnail cache

• [x] Create thumbnails and save them to cache folder
• [x] Thumbnail should be clearable

[LancerComet]

FileMeta part is done

[LancerComet]

Thumbnail caching is done in 0.13.0