File manager improvements

* Refactor FileDialog and HightlightDialog into ButtonDialog
* Created a new ButtonDialog when a file in the History dialog is hold, offering an option to delete the history entry. It is useful to do that inside the reader e.g.
   * If the book file does not exist anymore in the file system because the user removed it.
   * If the user just wants to clean up the history menu, by removing entries for books whose reading was finished long ago.
* When pasting a file to a destination, check if the destination is a directory or a file. If it is a file, the current behavior of pasting the file in the same directory as the current file (i.e. the current directory) is preserved. If it is a directory, paste the file inside the selected directory. IMHO this is a more intuitive behavior.