Improvements in object panel and drag/drop functionality

This update enhances the object panel's drag/drop functionality and improves the overall user interface.

Changes

• Objects panel drag/drop now supports multiple objects.
• Object panel layer now has a new collapsible system.
• Object panel tree design was improved, now it properly represents the structure.
• Object panel drag/drop behavior was improved and now allows dragging after the last element.
• Object panel layers now can be renamed individually.
• Parent object layers now indicate if a child object is selected (even if they are collapsed).
• Improvement of the way that objects are attached (parenting).
• Fixed issue of transparent images being converted to JPEG when they were bigger than 2048px.
• Fixed issue of grouping with CMD+G.
• Fixed issue with camera aspect ratio on view resize.
• Fixed issue of materials not being updated when switching between perspective/orthographic.
• Fixed issue when nudging 1 px unit with arrow keys.