Add project tree navigation shortcuts

[vkbo]

Summary:

This PR adds the following keyboard shortcuts for the project tree:

• Shift+Up and Shift+Down navigates up and down the project tree between sibling elements under the same node. Navigation stops at the boundaries. This is different from normal Up and Down navigation, which moves between rows and ignores the tree structure. The new functionality is mostly useful on the higher levels as you can skip all expanded child items.
• Shift+ Left navigates to the immediate parent of the selected item, stopping when reaching a root item. This is only marginally different than the Left key which will alternate between closing an item and moving to the parent, depending on the item's state.
• Shift+Right navigates to the first child of the item, expanding it in the process. This too is only marginally different than Right, but it combines expanding and moving in one keypress.

Related Issue(s):

Closes #1348

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