Behaviour when setting answers directory

[jgat]

When configuring the tutorial/answers folder (on first setup), there are some problems when the answers directory is set:

• if the user selects an answers directory which doesn't exist, then it is not created, and the user receives an error when trying to save files. Better behaviour would be to create the directory at that time.
• if the user selects a file for their answers directory, it should show an error message and prompt them to choose a directory.

On a similar (but less critical) note, if the answer directory is removed (or if ~/.mptrc is edited to point the answers directory at a nonexistent path), the error message when trying to save is not entirely helpful - a better solution would be to prompt the user if they would like to create the answers directory at that location, change the answers directory, or cancel the save operation.

[sapi]

For this, it might be worth looking into whether tkfiledialog permits you to set a directory mask (so that the user can only select directories).

Of course, it'll still be necessary to sanitise the input, as the user can type what they like into the textbox. (Alternatively, that could be made read only, but then it wouldn't be possible to automatically create a new directory.)