Add initial support for git-integrated export

[mmzmusicnotes]

• When files are exported, record the modified dates of all files changed at time of export - this should be every file in the project for first export or exports without fast save enabled, and only modified objects when fast save is enabled
• On subsequent export, files may be automatically overwritten so long as date matches and there are no pending changes (a "pending change" for this purpose is a file modified and unstaged, staged and uncommitted, or committed, which has not been updated by a previous import according to its modification date)
• If the HEAD commit has changed, or there are pending changes of any kind, prompt the user and warn them per file before proceeding (the warning may also have "yes/no to all" options)
• Changes in VCS version in .json headers can be ignored when exporting; investigate whether there are any other properties which should be ignored

(Note: If we adopt these requirements, update https://github.com/joyfullservice/msaccess-vcs-integration/wiki/Advanced-Git-Integration with them.)

[joyfullservice]

The git integration aspect seems to be tabled at this point. (To summarize, this is referring to using git to determine which files have changed, rather than referencing file modification dates.) We can reopen these issues in the future if there is interest in developing them further, but I am closing them out for now so we don't clutter up the issue pool with items that have no foreseeable completion date, and no developers actively pursuing them. Also closing #102, #103, #104, #105.