Catch crashes and offer to report them

[nagisml]

@AgreeDK this is a proposal. Carefully judge provided options

Uncaught exceptions went to stderr, which a windowed build doesn't have, so they were lost, and a hard crash left no trace the user would find.

• sys.excepthook / threading.excepthook log the traceback and show a "OpenSAK ran into a problem" dialog (Continue / Quit). Worker-thread exceptions are handed to the GUI thread through a queued signal. The same error shows the dialog only once per session.
• faulthandler writes native crashes (segfaults in Qt/WebEngine) to crash_native.log.
• A QLockFile session.lock is removed only on a clean exit. At the next start a lock left by a dead process means the last session crashed, and the user is offered a report with the native dump and the tail of opensak.log.previous. A second running instance is not taken for a crash.
• The dialog shows the exact report text and can open a prefilled GitHub issue or a mail to ....@opensak.com, copy the report, or open the log folder so the log can be attached. Nothing is sent automatically; home paths, the user name, tokens and e-mail addresses are removed first.
• New strings in all languages; tests in test_crash_report.py.