[Feature Request] Add merge dialog

[Heiss]

If there are changes on both ends, ask the user what should be done. Add an option to set this behaviour in settings for all merge conflicts. The dialog should have an "always" option, so the settings will be set automatically on first occusion.

This is possible, because we have the cache #12 implemented, now. And so we have the opportunity to find out, if there are multiple changes. But not with the current implementation... We need to implement versioning in cache on our own, so we can see, that there were changes on our end, too. And so the cache cannot only maintain an overview over the changes of vault, but for the two latest versions of vikunja, too.
This will increase the needed storage.

Needed versions

• synced version, the latest state of task which was synced last time to / from vault (stored)
• updated version in vault, so we know, that there are outstanding changes in vault (stored)
• current version in vikunja, if it has a newer updated date then synced version, we know that there are outstanding changes (pulled from vikunja while sync)

dialog

Shows which differences were found, when the vault and vikunja version was updated and the options to act with an always checkbox, which saved the selected option to settings.