Enable using differently named layers and fields

[bogind]

The plugin currently only supports styling layers with the same name as the supplied .lyrx file and with the same column names,
This is solvable through renaming the desired layer to the name and adding virtual fields.

suggested solution:

1. Detect style layer name and allow to ignore it
2. Detect used field names and add as many field name selectors as needed

This is the cause for #5 and other similar errors.