ui: add opt-in setting to remember password

[norwnd]

Consider a button on settings screen that will allow:

• dexc to remember password across restarts
• if that's not viable ^ then maybe have just an option that swaps default for remember password field in UI from "non-checked" to "checked"

this is useful during development/testing, and for cases when risks don't concern the user (if he runs dexc in "safe" environment, he doesn't need to enter all those passwords every time). It doesn't sacrifice "security-oriented by default" either.