Various bug fixes for v5-beta1

[jevenski]

Fix legend text overlapping by adding a non-transparent background to it. Apply some minor display adjustment for MaterialDesign theme as well.

[jevenski]

I mixed a few more bug fixes into this pull request, so we can speed up the code review process. Correct me if it's better practice to aim one target per pull request.
These new commits fixes the "Save" button on the spectrogram dialog and the create-torrent dialog. Old spectrogram save fix didn't get it right yet. The "Save" button was added every time we create a spectrogram, and it's still there after we close the dialog. So if we create 100 spectrograms, there will be 100 "Save" buttons. As for the create-torrent dialog, the "Save" button was added as visible, while it should be added as hidden.
In the utility classes clean-up commit, I added another action to center any open dialogs when the WebUI resizes. This could be useful when we switch between different display sizes when developing, or change device orientation when using ruTorrent in production.

[stickz]

Thanks!