Save note automatically

[jancborchardt]

Similar to the same issue on Android: aykit/MyOwnNotes#25

Notes are not saved at all unless you press the »Save« button. This leads to loss of notes very easily as you don’t always remember to press the save button.

Instead, notes should be saved in one or ideally all of these ways:

• when pressing the save button of course
• when going back to the notes list
• 300ms after finishing typing
• when exiting or switching the app

[kh0r]

Good suggestions.
We wouldn't even need a save button after these changes.