Add an icons class and a few icons

[vkbo]

Summary:

This PR adds a custom icon engine for SVG icons, and a static icons class where the icon SVG is just saved in a QHash map. Four icons have been added for the existing toolbar buttons.

Related Issue(s):

Resolves #17

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
• Function descriptions are complete and understandable
• Only files that have been actively changed are committed