Fix style filters setting

[ntadej]

Fix style filters setting as Qt awkwardly detects QString as a list so it was iterated into a list of QChar. Also add the following improvements:

• Add ability to remove the filter easily by setting an empty expression.
• Add a FilterParameter helper class.

Fixes #104.

[ntadej]

Will exceptionally merge this now as iOS is failing due to storage issues. As no platform-specific code is touched, this should build fine also on iOS (and does, it fails on linking).

[kebekus]

@ntadej I guess I am a little late. Still: Thanks for fixing this! I can confirm that the fix works great for my software.