Add result count to search tool

[vkbo]

Summary:

This PR rewrites how searches are run in the editor. All searches are run in advance, and the number of results are displayed on the search tool. Looping through search results is thus just a matter of iterating through the list.

This PR also changes how the "Scroll Past End" feature works in the editor. The old implementation changed the margin of the document root frame each time the document margins were updated. This is technically a change to the document, and therefore goes on the undo stack. This is triggered each time the search tool is opened, so it is a rather annoying side effect as it pollutes the undo stack. The feature has been rewritten to add a static margin on the document fram when the document is opened, and the undo stack is then cleared. The margin is a fixed number of lines, and is a user setting in Preferences.

Related Issue(s):

Resolves #645

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