Added an option to clear text editor when pasting from the clipboard.

[veler]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• UI change (please include screenshot!)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Internationalization and localization
• Other (please describe):

What is the current behavior?

Issue Number: #562

What is the new behavior?

• Added a new option in the Settings allowing the user to clear the text box when pasting from the clipboard, so it replaces the content of the text box by the one from the clipboard.
• Fixed a bug where Base64 Image Encoder / Decoder doesn't update accordingly to the user settings changes.
• Applied the font settings in every text editor.

Other information

Quality check

Before creating this PR, have you:

• Followed the code style guideline as described in CONTRIBUTING.md
• Verified that the change work in Release build configuration
• Checked all unit tests pass