Add initial Debian packaging for stable-1.2

[jasp00]

This is the first step towards Debian integration, as stated in #4695.

[PhysSong]

Could you consider removing clang.patch? We don't add -Werror by default anymore. Also, I can't apply the old patch into current stable-1.2.
Also, CONTRIBUTORS are outdated in your patch.

[jasp00]

I did not explain that patches under debian/patches are not meant to be applied, since the package format is native.

Could you consider removing clang.patch?

It is only a reminder to add a clang pass on Debian sid.

We don't add -Werror by default anymore.

It is good practice to enable -Werror on sid. Specific errors can be disabled.

Also, CONTRIBUTORS are outdated in your patch.

It is just another reminder.

[trebmuh]

Suggestion to remove the debian/menu file following this Debian bug report which have deprecated the menu file.

[jasp00]

If there is no more feedback, I will merge this and continue the integration.

[PhysSong]

I forgot to mention that debian/lmms.xpm should be updated according to #4519.