Add app shortcut for composing email

[philipwhiuk]

Expected behavior

Be able to long-press on the K-9 app for a short-cut to compose an e-mail.

Actual behavior

Nothing. We don't have any shortcuts

Steps to reproduce

1. Long-press on the K-9 app

Environment

K-9 Mail version: 5.203

Android version: 7.1.1

Account type (IMAP, POP3, WebDAV/Exchange): IMAP

Developer Notes

Requires API version 25

[GoneUp]

Wouldn't it also be useful if the user could define a few favourite contacts that are directly used as recipient? Or we provide shortcuts to either the most used or the last recently used addresses. The problem with this would be that they need to be updated to be in sync with the database.

[cketti]

Depends on #2110

Note: You can get most of this functionality today by using one of the numerous "email shortcut" apps.

[philipwhiuk]

Needs a better icon and there's an package issue regarding the debug suffix but this isn't so complex (once #2110 is fixed). See https://github.com/philipwhiuk/k-9/commit/a712d9e34f267052e47ba924121965d4e550efc3 for the changes needed.

[532910]

As I said in my dup #3919 I'd like to see new letter shortcut in that list.

[cketti]

Developer documentation: https://developer.android.com/develop/ui/views/launch/shortcuts