Activate `lexical-binding` in a few more files.
Use `advice-add` rather than `defadvice`.
Fix some compilation warnings.
Prefer #' to quote function names.
Adjust `Package-Requires:` accordingly.
When using this multiple-cursors.el repository as a submodule and
using compilation from .el to .elc, git will report the .elc files as
untracked changes in the submodule. Ignoring *.elc files fixes that.