ledger-mode: Use easymenu to remove boilerplate

Among other benefits, this lets us define the menu entries from top to
bottom, rather than in reverse.
ledger-mode: Add namespace prefix to highlight-overlay

make-variable-buffer-local is a top-level form: this commit moves that
call from ledger-mode to next to the variable's declaration.
ledger-mode: Remove duplication in ledger-set-{year,month}
ledger-mode: Add missing argument to ledger-delete-current-transaction
ledger-mode: Fix a number of byte-compilation warnings
Don't default fill-column to 8 in Emacs
ledger-mode: Also bind g to ledger-report-redo, as is standard elsewhere
Pop the new schedule buffers
ledger-schedule-upcoming take argument.

One can forget to run ledger-schedule-upcoming for some time. Then there
is the risk that ledger-schedule-look-backward is too short. This will
open the possibility to use a different file, or to change the period
backward or forward for this kind of case.
Merge pull request #223 from vanicat/t/schedule-with-args

I should have done this a long time ago, but I got too lazy.  Thanks Remi!