Add undo/redo support for the pattern editor

[tema5002]

This pull request introduces a ring-buffer-based undo system (ft2_undo.c/h) covering: note edits, pattern insert/delete/length changes, song length/loop start, BPM/speed, channel count and global volume.
Related edits across the pattern editor, position editor, and trim tool now push undo records for their changes, grouped with undoBeginGroup/EndGroup where an action touches more than one cell.
Bound to Ctrl+U and Ctrl+Q in the keyboard handler.

Also a few minor changes I wouldn't open a small not-too-useful PR for:

• Make Linux build scripts executable by default (avoids chmod +x <file.sh>) and able to run in just POSIX sh, not just Bash.
• Add a home button to diskop. that redirects to the current user's home directory when compiling for MacOS/Linux. Also removed the "/.." directory entry.

The help section dedicated to keybinds was not updated.
From what I've tried it works perfectly fine, everything tested on a GNU/Linux system.

If something needs to be done (like renaming some functions) I can push a follow-up commit.