Issue #438: Editor cut/copy/paste

[vkbo]

This PR adds:

Document Editor

• Cut and Copy to the context menu when a user has made a selection.
• Paste, Select All, Select Word and Select Paragraph to the context menu.
• Spell checking is added to the same menu if any suggestions are available.
• Select Word and Paragraph selects the word/paragraph under the pointer, not the cursor as the equivalent main menu entries do.

Document Viewer

• Removes the built-in context menu and replaces it with the Paste, Select All, Select Word and Select Paragraph features from the document editor context menu.

These changes makes them consistent (the built-in menu looks different than the other menus) and the editor and viewer are now consistent in features and functionality with each other.

Tests have been added to check that the selection functions work in both panels and the context menu can be built without crashing (no other functionality checks are made on the context menu itself).