This patch adds support for key accelerators and -underline.
It parses the given button labels for ~, underlines the character
following it, and adds a binding of <Alt-character> for the
respective button. Strings for labels are cleaned of any ~ which
has the potential for regressions.

This is actually annoying and not helpful.

If no netlist is given, we cannot validate... but this is an expected use case
not an error.
Likewise we tried to avoid to validate if no driver was selected but failed
to do that correctly (the driver path is (probably) always defined but empty
if none was selected).

 - 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.

 - add detailed installation instructions
 - refine appendix
 - various smaller refinements and style changes

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.

 - add captions and change references
 - refine formatting

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.

Wrap everything into an eval() to contain faults.
Fail to succeed for unknown reasons. Possibly because of the impossibility
to stop Perl:Tk from going berserk when the function does not set up
the widget correctly before returning?

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.