Don't cast function pointers; calling functions via pointers of
incorrect types is not guaranteed to work. Instead, define the
functions with the desired types, and make them cast the incoming
parameters. Or define wrapper functions if the return types don't
match.
really_exit_prog wasn't being used outside src/dialogs/menu.c,
and I had to change its parameter type, so it's now static.
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.
