last version of files in epub_content (validated by epubcheck 3.0)
Review and small fixes for german Translation

reworded some translations
specify context menu item to epub files only (#249)
context menu item now with i18n_add_book
i18n_validate is now used in library.html
i18n_add_book is now used in library.html
i18n_cancel is now used in library.html
i18n for the message 'loading library' in library view
Reset fs-toggle-btn when exiting fullscreen mode with Esc key
Merge pull request #269 from vincent-gros/master

updated epub_content | specify the context menu item
Merge pull request #270 from shorshe/patch-1

Review and small fixes for german Translation
Merge pull request #271 from h2so5/synchronize-fs-toggle-btn

Reset fs-toggle-btn when exiting fullscreen mode with Esc key
Merge remote-tracking branch 'upstream/master'
just a typo
updated messages.json (pt-BR)
it was an underscore (from pt-BR to pt_BR)
rewrite messages.json for pt-BR to respect the original order
Merge pull request #274 from vincent-gros/master

i18n - use unused messages (add/cancel book, loading library, etc.)
prevent dubplicate manifest item ids

this prevents generation of duplicate item ids when manifest item ends in
number. inserting an underscore makes sure that new id's stay unique
(e.g. html2 pos 26 and html22 pos 6 will become html2_26 and html22_6
instead of both html226)

fixes #262
Merge pull request #276 from artm/unduplicate_models

prevent dubplicate manifest item ids