wx Tasklist: re-implement filters with a navigator

Addresses #227
Reset some menu accelerators
wx Tasklist: add menu items for controlling the navigator

Addresses #227
wx GUI: Reset the menus only when the top-level ones are closed
wx Tasklist: simplify code
wx Tasklist: Let set the first day of the week

Addresses #227
wx Tasklist: Remember navigator show status

Addresses #227
wx Tasklist: rename variable
wx Tasklist: Let choose which navigator buttons to show

Fixes #227