Connectivity view (instead of spamming the user with error_network when sth fails) (#2319)

See https://support.delta.chat/t/discussion-how-to-show-error-states/1363/10 <!-- comment -->

It turns out that it's pretty easy to distinguish between lots of states (currently Error/NotConnected, Connecting…, Getting new messages… and Connected). What's not that easy is distinguishing between an actual error and no network, because if the server just doesn't respond, it could mean that we don't have network or that we are trying ipv6, but only ipv4 works.

**WRT debouncing:**

Sending of EVENT_CONNECTIVITY_CHANGED is not debounced, but emitted every time one of the 3 threads (Inbox, Mvbox and Sentbox) has a network error, starts fetching data, or is done fetching data.
This means that it is emitted:
- 9 times when dc_maybe_network() is called or we get network connection
- 12 times when we lose network connection

Some measurements: dc_get_connectivity() takes a little more than 1ms (in my measurements back in March), dc_get_connectivity_html() takes 10-20ms. This means that it's no immmediate problem to call them very often, might increase battery drain though. For the UI it may be a lot of work to update the title everytime; at least Android is smart enough to update the title only once.

Possible problems (we don't have to worry about them now I think):
- Due to the scan_folders feature, if the user has lots of folders, the state could be "Connecting..." for quite a long time, generally DC seemed a little unresponsive to me because it took so long for "Connecting..." to go away. Telegram has a state "Updating..." that sometimes comes after "Connecting...".

To be done in other PRs:
- Better handle the case that the password was changed on the server and authenticating fails, see https://github.com/deltachat/deltachat-core-rust/issues/1923 and https://github.com/deltachat/deltachat-core-rust/issues/1768
- maybe event debouncing  (except for "Connected" connectivity events)

fix https://github.com/deltachat/deltachat-android/issues/1760

src/events.rs

@@ -185,21 +185,6 @@ pub enum EventType {
     #[strum(props(id = "400"))]
     Error(String),
 
-    /// An action cannot be performed because there is no network available.
-    ///
-    /// The library will typically try over after a some time
-    /// and when dc_maybe_network() is called.
-    ///
-    /// Network errors should be reported to users in a non-disturbing way,
-    /// however, as network errors may come in a sequence,
-    /// it is not useful to raise each an every error to the user.
-    ///
-    /// Moreover, if the UI detects that the device is offline,
-    /// it is probably more useful to report this to the user
-    /// instead of the string from data2.
-    #[strum(props(id = "401"))]
-    ErrorNetwork(String),
-
     /// An action cannot be performed because the user is not in the group.
     /// Reported eg. after a call to
     /// dc_set_chat_name(), dc_set_chat_profile_image(),
@@ -330,4 +315,11 @@ pub enum EventType {
     ///     (Bob has verified alice and waits until Alice does the same for him)
     #[strum(props(id = "2061"))]
     SecurejoinJoinerProgress { contact_id: u32, progress: usize },
+
+    /// The connectivity to the server changed.
+    /// This means that you should refresh the connectivity view
+    /// and possibly the connectivtiy HTML; see dc_get_connectivity() and
+    /// dc_get_connectivity_html() for details.
+    #[strum(props(id = "2100"))]
+    ConnectivityChanged,
 }