Add Special Characters for to Write Activity

[AdityaShome]

This PR adds the ability to insert special characters into the Write Activity document editor.

Changes

• New widget: Implemented SpecialCharactersWidget that displays categorized special characters in a tabbed interface.
• Toolbar integration: Added special characters button (omega symbol) to the Text toolbar.
• Character categories: Organized characters into useful groups (Greek letters, currency symbols, math operators, and accented characters).
• Usability features: Added "Recently Used" tab to track frequently used characters and tooltips showing Unicode character names.
• Compatibility: Ensured special characters work properly with AbiWord's text handling system.
  

Implementation Details

The implementation focuses on simplicity and compatibility:
Carefully selected character sets that render correctly in AbiWordactivity
Compact UI design that fits within Sugar's toolbar palette system
Direct Unicode insertion without requiring complex encoding transformations

Testing

Tested with various document formats including plain text and ODT to ensure special characters display and save correctly.