Add context menu to Books

[babluboy]

Add a context menu for each book in the Library View to show:
1- Update Cover Image : Should open a window and allow user to choose a image
2- Update Book Title if the default cover is being used: Text Entry with current title populated
3- Add tags to a book : Text Entry with current tags populated
4- Add rating for the book : Usual 5 stars to choose from

[babluboy]

Hacked a bit to add the context menu to the books in the the library view

[babluboy]

As the first implementation of the context menu, added functionality to search by tags using the search bar in the header. The search bar default text has been updated to include "Tags" along with Title and Author.

Raised separate issues to track the following:
1- Make better UX options to search by available tags #45
2- Update the cover image in the library as soon as the update is done in the context menu (At the moment the changes to the cover takes effect only after closing and re-opening Bookworm) #47