Update v2.0.0 - Weighted Centering

Changes:

• [Feature] Weighted Centering (suggested by @not_surt)
• [Improvement] From Aseprite v1.3-rc2 - add a new submenu for the Center options, and add centering only on X or Y axis as menu options (previously only available as key bindings)
• [Improvement] Add default keyboard shortcuts
• [Fix] Block centering on locked layers