Thus, each caller must now choose the accelerator key and declare the
accelerator contexts (i.e. menus) to which it may add the command.
Also, use only one context for tab_menu.
These changes fix the following bugs in accelerator conflict detection:
* "~Pass frame URI to external command" may be displayed together
with "Pass tab URI to e~xternal command", but that was not
declared.
* "Pass link URI to e~xternal command" was declared as being in
the tab menu, but it is actually displayed in the link menu.