Improvements to the "add a book" UI

• Use ellipses in action name
• Add an icon to the action
• Use a placeholder message telling user to add books if library is empty
• Add a name filter to only show epub, fb2 etc files
• Don't show the "Add Book" action in the toolbar if there is a placeholder message — less redundancy

Also had to change .gitignore to ignore .cache/, otherwise I'd be committing a bunch of clangd cache with this.