Use native PyQGIS API functions instead of custom parsers where possible

[kannes]

Via #34 (comment)

I wonder if it isn't possible and preferable to use the PyQGIS API to manipulate bookmarks?

https://qgis.org/pyqgis/3.38/core/QgsBookmarkManager.html

This also applies to other things.

If you work on this, make sure to check if the minimum supported QGIS version of the plugin supports the API functions you want to use. It would be best if we can keep supporting older versions, at least as sources.