New Features and Bug Fixes

• New color picker for Fills and Borders.
• Zooming with mouse wheel follows the position of the cursor.
• Fix loading Artboards background color when opening a file.
• Fix numbering of newly created items.
• Fix Gtk-CRITICAL warning when opening the Export Dialog.
• Fix flipping items inside Artboards.
• Fix crash when dropping flipped items inside Artboards.
• General code improvements and optimization.