Improvements to text field item and script menu

[Vankata453]

Improves the text field and script line menu items:

• Support for going back in text and inserting/deleting characters from the current position. ("Backspace" for deleting characters in front of the cursor, "Delete" for deleting behind it.)
• Cut and copy support for whole lines. [Ctrl+X, Ctrl+C]
• Paste support (new-line characters are converted to spaces). [Ctrl+V]
• Undo/redo support (restricted to 1 step for both). [Ctrl+Z, Ctrl+Y]

Some tiny improvements take place in the script menu as well:

• Ability to duplicate lines. [Ctrl+D]
• Fix for a bug, where the menu doesn't change its size when a script line is deleted.

The int field and float field items were also refactored. They now inherit the text field item.

This PR aims to provide a partial fix to #936. Partial cut/copy, and paste support are added, but not selecting text.