Customizable display

[ByK95]

Creates a interface so end user can modify or create own update display. Converts Notification , Snackbar , Dialog into classes

adds example of custom toast display

I think functions in AppUpdater like

• https://github.com/javiersantos/AppUpdater/blob/ca6bcd2495095d6965ddf15d6a6fea6e80fab35c/library/src/main/java/com/github/javiersantos/appupdater/AppUpdater.java#L311

• https://github.com/javiersantos/AppUpdater/blob/ca6bcd2495095d6965ddf15d6a6fea6e80fab35c/library/src/main/java/com/github/javiersantos/appupdater/AppUpdater.java#L74

should be moved into own display classes.