Resolve "Use Qt Resource System for all the extra files we ship"

This addresses all the remaining points in code where external resources were being loaded. They are now all getting embedded with the code itself, using Qt Resources.

It also partially addresses #37 (closed), with checkplugin moving to use QTextDocument. Note I was going to actually push it separately under #37 (closed), but at some point reverting the changes from this branch was just PITA, so I kept it as is. I checked all the templates and the subset of the HTML/CSS that QTextDocument supports covers all the html/css elements used, so IMHO there's no need to use QWebKit/WebEngine in the case of this plugin.

The biggest change here is the check printing plugin. One thing I did not look into too deeply was the migration of the previous template configuration setting. I could, theoretically, check if the previous template was a "standard" location and replace it with the built-in "qrc://" path, but I cannot, realistically speaking, determine all of the absolute "standard" paths possible on all systems, and using a relative path matching is obviously not acceptable, as any user could have saved their custom template at a location that would yield a false-positive match. Another idea I have is to check if the configured file still exists on file system and if not, to have KMM load the default one. I think this should be good enough for the migration. EDIT: implemented the latter.

Closes #33 (closed) Relates to #37 (closed)