Clean account types

### Actual behaviour
The app handles account using 4 (!) different formats:
- `Account` - obviously
- `OwnCloudAccount` - wraps account, platform API + adds some own API
- `String` - needs to get account by name
- `int` - get by hash code?

Of course the conversion, getters, etc code spills to code that actually uses account data.

### Expected behaviour

- 1 (one) data type to deal with.
- data type provides all required API to query about account capabilities (this logic taints the UI heavily)
- no ad-hoc conversions are required to manage app accounts

### Solution
- Immutable `NextcloudAccount` type providing all the functionalities.
- Current account available directly in `UserAccontManager`, avoiding expensive queries to platform `AccountManager`
- No references to `Account` (long term, I guess)

### Alternatives
- sanitizing `OwnCloudAccount` but it's not a data type as it brings a lot of platform services with and does not provide too much functionality right now