Dynamically load list of themes

[fruzitent]

As part of efforts to implement hot-reloading, themes should be scanned each time the settings window is opened or even a selector drop-down is accessed, rather than one time when the application is launched.
https://github.com/Chatterino/chatterino2/pull/4570/files#diff-28b86349b423ea5130d327d9c350d0be262e92b26fc45d496d796e2b0370dd38R158
#4718