HTML-Export: Add "external css" option

exports html source code with reference to an external style sheet.
Only makes sense with
- document="full"
- format="html"
Update File menu mnemonics

Not all have been set, but significantly more than before
HTML export: Adapt colorize.py

Fix colorize.py to reflect the modifications done to highlight2html.py
Export-dialog: bind options to QSettings

Export options are read from QSettings (group "export").
Functions to save them (automatically) are implemented too,
but still lack an interface

Needed is one setting in the Preferences dialog: "export/autosave_settings")
Export-dialog: Empty clipboard on cancel
Export-dialog: Make it QSettings-aware

Export options are now stored with the global QSettings object.
They are persistent throughout the session,
can be saved explicitely (from the "Export source" dialog) or
implicitely (Checkbox in Preferences->General)

The Dialog itself is a mere dummy yet. At least it shows
disabling of the "Save" button, Cancel and OK work too
(Cancel clears the clipboard)

The export options can't be edited in this intermediate state.
Default values can be set in export/__init__.py,
but they will be overwritten through the Settings.