Create an export dialogue

[yehia2amer]

There are currently two export options: “Export Selection” and “Export Selection Without Notes”. The first exports a tab-separated value file containing the words marked as “unknown” with any notes in a column to the right of the words. The second of these excludes any notes. These two options should be replaced with a single option to bring up an export dialogue. The dialogue should include these options along with the option to select any combination of the words marked as “known”, “unknown” or “unseen”. This will allow, for example, a user who wants to export new vocabulary that they want to learn to a flashcard program by selecting the option for “unknown” words but leaving “known” and “unseen” unselected. A user wanting a list of all of the vocabulary, regardless of whether they know the words would select all three options: “known”, “unknown” and “unseen”.

As Discussed with @AdamCarroll in #41