GTK3 migration - RemminaStringList to grid layout + bugfixing
Fixes #324
* Manually merged commit '9868ff1500e075ff228feaaf8e1cfd7d6c95b2b9'.
* Backported GTK+2 support for RemminaStringList

Conflicts:
	remmina/src/remmina_pref_dialog.c
Merge commit 'b4122a1a623d4d0066927b9bb07e2417bebc8f77'
Removed HTML tags to avoid gettext conflicts
GTK3 migration - remmina_chain_button to grid layout
For RemminaChainButton, use GtkGrid on GTK+3 and GtkTable on GTK+2
Update README