Enable showing image title/hover text on long press

Listens for long click in webview, if it is on an image, gets its url,
looks for the title tag before or after the url and shows an alert
dialog if found.

Conflicts:
	app/src/main/java/fr/ydelouis/selfoss/fragment/ArticleFragment.java
Small refacto
Merge branch 'show-image-title' from bravekarma