Fix and improve quest editor (Rebased)

[KatatsumuriPan]

This is rebased branch of #98.

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This is a response to #97.

What's Changed

• Restored quest description editor gui (moved to a new class).
• Both the editor gui screen (suitable for preview) and the editor window (suitable for writing) are available.
  You can open the window with the button.
• "Juse Close" button allows to close an editor gui screen while keeping the window open, and vice versa.
• It is still possible to have multiple quest windows open at the same time.
• Added line numbers on the editor window.

Bug Fixes

• Quest name and description are now correctly updated when the text field has focus.
• Scrollbars and component sizes of the editor window was fixed.
• The text formatting buttons now correctly replace the selected text in the editor window.

This branch contains the commits of #95.