TopLevelView Drilldown for for Icinga Web 2

What's Changed

This will be the final version with support for Icinga Web monitoring module. Future versions will use IcingaDB Web.

Implemented enhancements:

• Improve CSS for dark/light theme
• Add confirmation dialog to deletion button
• Add Icinga Web CSS classes to delete and cancel button
• Update codemirror5 to latest version and add code folding
• Update README to include php-yaml requirement

Fixed bugs:

• Fix CSS issue with node collapse icon
• Change unsaved work warning position to avoid breaking the layout

Full Changelog: https://github.com/Icinga/icingaweb2-module-toplevelview/compare/v0.3.3...v0.3.4