macosx: Fix minimum sizing of library split views

This prevents some nasty crashing/bugs caused by letting the main view pane become too small

After fix:

Screen_Recording_2023-04-14_at_15.34.58

Before fix:

Screen_Recording_2023-04-14_at_15.37.43

Signed-off-by: Claudio Cambra developer@claudiocambra.com