Add the ability to save a file's settings as default

[TolikPylypchuk]

Preferences contiain the default settings among other things. If the user changes a file's settings (e.g. by adding lots of tags), then in order to make these settings the default ones, they will need to duplicate all the work. Instead they should be able to simply mark the settings as default by pressing a button.

[TolikPylypchuk]

The app will be completely rewritten as a web app, thus all opened issues are being closed.