* add 'Add to Home Screen' menus for webxdc apps
* make adding shortcut basically work
* avoid starting webxdc from foreign accounts
* recreate back stack, if possible
according to
https://developer.android.com/training/notify-user/navigation :
"When you start an activity from a notification,
you must preserve the user's expected navigation experience.
Tapping Back should take the user
back through the app's normal work flow to the Home screen [...]"
same seems to be true for shortcuts using the same class for constructing
the back stack.
the back stack is reconstructed for API 26 (Oreo, 2017) or newer,
for oder API `requestPinShortcut()`
only uses the top-level activity for the shortcut.
working around that is probably possible, but would be quite some effort in an
already complicated area and would result in bugs that are not so easy to see.
also, on these old androids, users are probably used to the no-back-stack
behaviour.
* show a warning if app was deleted
* add 'Add to Home Screen' also to gallery
beside chat- and webxdc-activity,
this is the third place where the webxdc is listed.
as adding may result in dialogs from the launcher,
we allow the option only for a single selected webxdc,
as otherwise a series of dialogs would pop up.
* show 'Done' toast short, this is what we are doing at all the other places when showing 'Done'
bjoern