- Change FIJIModalDialog to handle markup text with URLs.
 - Add a unified about text in a file.
 - Create a central show_about() and use that in Setup and EE.

TODO: docs/

Reuse code of show_documenation to create a generic
cross-platform open function.
Also, get rid of the extra help button in FIJI Setup and FIJI Download GUI.

This also makes delete_mw superfluous in FIJISettingsViewer and FIJITestsViewer.
'mw' always was useless because there exists a Mainwindow() in all widgets.

They are not useful in git. If need be they could be implemented by a
fudge/clean filter with git attributes.

We can check if the main window still exists with Exists() instead.
This may also have fixed the problem with joining all threads...

No other changes were made.

Put download_worker code into eval and report its demise to the GUI
via $queue_to_gui.
This allows us to abort safely on fatal errors like missing packages (if
nothing else interferes... I am looking at you Tk!)
To that end install an afterIdle event handler that checks for said
message, displays an error dialog and exits the GUI when the user has
acknowledged the problem.

Empty last parameter lead to warnings if other strings are empty.

Adresses Issue #30

Adresses issue #39

from resource calculation

Images.

Re-enable joining the threads correctly.