adds an "interactive download mode" option to replay "saved requests"

when checked:

* each "saved request" that is selected for replay
  will trigger a file picker dialog,
  which allows the user to select the file path
  to a file that is the result of a partial/incomplete download.

when unchecked:

* each saved request that is selected for replay
  will begin being saved to the "default download directory",
  as selected in the addon preferences.

status:

* this feature works beautifully
* the one issue I have is deciding what action the tool should take
  when a user clicks "cancel" in the interactive file picker..
  * either the download could proceed using the "default download directory"
  * or the particular saved request could be skipped and its replay cancelled

to do:

* the next commit will add an additional checkbox to the "replay" dialog window,
  which will allow the user to select the fallback behavior
  (when cancel is chosen for locating a download file path in interactive mode)