Add a text import tool

[vkbo]

The import text feature that currently exists is very basic. A proper import tool can improve on this, and could include the following features:

• Allow selecting expected input encoding when importing from file
• Allow pasting text into a text box that can handle rich text formatting, and then try to preserve some of the formatting
• Chose whether to create a new document in the project tree, or insert the text at the cursor position in the current document