Allow opening BAK files from sidebar

Enhancement Summary

The sidebar has a section called “My projects” that lets you browse the projects directory of LMMS and open projects.

Double clicking a file will open it.
However, LMMS can only open MMP (LMMS project) and MMPZ (compressed LMMS project) files. It cannot run BAK (backup) files.
LMMS should be able to open (mmp and mmpz) BAK files when opened from the sidebar. (Note that some other programs also use bak, so it will need to be ensured that the file in question is a .mmp.bak or a .mmpz.bak and not any other bak.)
(Not restricted to “My projects,” also extends to “Home” and essentially any sidebar window that displays directories.)

Justification

Convenience of opening a BAK file as easily as opening an MMP file without having to use open dialogue, rename the file, or manually opening through file explorer.