Fixed Text File Editor losing unsaved changes when rotating screen

[Burgerpower01]

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What is it?

• [X] Bugfix
• [ ] Feature
• [ ] Codebase improvement

Description of the changes in your PR

• Changes in ReadTextActivity:
  • Added onSaveInstanceState() to save edited text and restore it when Activity is reloaded
  • Added parameter savedInstanceState to checkIntent() to access the unsaved text

Before/After Screenshots/Screen Record

• N/A

Fixes the following issue(s)

• Fixes #106

Acknowledgement

• [X] I read the contribution guidelines.