First draft of converting ListViews from SoloMainActivity to fragments

* Renames ids in layouts
* If the ListView is empty, an "emptiness" message is shown
* Some refactorization due to better understanding of how the fragments work
  and communicate.
Comments all stuff related to old SoloPlaylistsActivity layout
Works in progress with fragments
Goes on with f**king fragments
Adds Groups and Solo in menu
Adds forgotten GroupsActivity start in AbstractMenu
Hides menus if useless (solo in solo, groups in groups)

The Solo menu entry is hidden in the solo activities. The same rule stands for
the groups activities.
FIX missing group stats

Many fuzzy stuff about Fragments.
I think I hadn't seen much in the lists because of a not so stable android
layout template from dev.android.com.
I'll start again from null tomorrow, and will work hard to make these fragments
display some content. Hope it'll go fine...
Remote playlists fragment displays the remote playlists :D

This doesn't sound really exciting, but for me it's a great achievement ;)
It's not about displaying playlists, it's about understanding these fragments
and getting them displaying the content I want the way I want.
Merge branch 'dev/solo'

Conflicts:
	AndroidManifest.xml
	res/values/strings.xml
	src/ch/epfl/unison/ui/GroupsMainActivity.java
	src/ch/epfl/unison/ui/SoloPlaylistsActivity.java
	src/ch/epfl/unison/ui/SoloTracksFragment.java