Improve back button behavior and preserve main activity state after rotation

[Fs00]

Here's what's new with this PR:

• SearchConfig has been renamed to SearchState and moved to base.main package (since it's used only for main activity)
• the current folder is now stored in SearchState in a nullable property called currentFolder instead of using only the first place of folders list (which I've deleted). That list was just an overcomplication (you can't search in two folders at the same time), since only its first place was used
• when the user is inside a folder and presses back, the app just exits the folder without resetting the whole search state (e.g. if now you're in Archive -> Folder, when you press back you get back to Archive, not to Home anymore)
• when the user selects a tag from the main menu, the search state is not resetted anymore. This behavior could be confusing and caused bugs if the user was inside a folder
• main activity state is now fully preserved after screen rotation
• various code refactorings aimed at duplication removal

This is an impactful PR on the codebase. The sooner you merge it, the lesser is the chance to get merge conflicts.

[Fs00]

Here's a demo of main activity state preservation: https://streamable.com/sbf5o

[Fs00]

@BijoySingh PLEASE look into this PR as soon as you can so that we can avoid other conflicts and rebases. Can we get this merged before the next release?