use app-term instead of webxdc-term; add "About Apps" to three dot menu (#4025)

* deprecate 'what is webxdc?' string"

it is used when an in-app is on the screen (on desktop),
however, instead of helping, immediately raises another question,
as the user just did not know about webxdc at all at that point
(we are not using the term in the apps at all).

better just use the string 'Help' and open
offline help at #webxdc - the entry is also just improved :)

* add 'help' to app-menu

* just 'help' may be confused with 'help about that app', be little more specific

* update CHANGELOG

src/main/java/org/thoughtcrime/securesms/WebxdcActivity.java

@@ -296,6 +296,8 @@ public class WebxdcActivity extends WebViewActivity implements DcEventCenter.DcE
     if (itemId == R.id.menu_add_to_home_screen) {
       addToHomeScreen(this, dcAppMsg.getId());
       return true;
+    } else if (itemId == R.id.webxdc_help) {
+      DcHelper.openHelp(this, "#webxdc");
     } else if (itemId == R.id.source_code) {
       IntentUtils.showInBrowser(this, sourceCodeUrl);
       return true;