Replace the edit item dialog

[vkbo]

Summary:

This PR adds a simple text editing dialog for editing labels. It behaves similarly to the QInputDialog, but with a few improvements to the layout.

Sine the context menu in the project tree now can change all settings of a project item, the old GuiItemEditor dialog has been removed. To preserve the possibility to edit items in the project tree without using the mouse, the shortcut Ctrl+. will now open the context menu on the selected item, just like in the document editor.

This change renders feature #997 obsolete.

Related Issue(s):

Closes #997

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed