Postpone instantiating modal windows

[yedpodtrzitko]

easy backport from #332 which could be actually nice for the upcoming release

• All the modal windows (Folders to Tags etc.) are always instantiated immediately, which takes time. This PR postpone them until it's actually needed, which speeds up the application start.

- also reuse UniversalDetector instance for files encoding detection.

[CyanVoxel]

Thank you for this! The UI initialization has been ripe for optimizations like this.

When testing the UniversalDetector however, I noticed that it appears to not play well with the multithreaded nature of ThumbRenderer. When running successive tests on a folder full of differently encoded text files, I was able to get it to consistently mix up the encodings producing a different spotty mix of unrendered thumbnails each time.

[yedpodtrzitko]

I reverted the last commit with UniversalDetector