tidy up store interface and fix selected

This pull request does two things: it adds a few methods for manipulating GtkListStore and GtkTreeStore objects and it fixes the `selected` method and adds `selected_rows` to mirror `gtk_tree_selection_get_selected` and `gtk_tree_selection_get_selected_rows` (which don't work with G_.select or G_.selected_rows, as far as I could tell).

The changes to the store methods are summarized below with - meaning dropped, + meaning added:

## store interface
```
push!(listStore::GtkListStore, values::Tuple)
unshift!(listStore::GtkListStore, values::Tuple)
insert!(listStore::GtkListStoreLeaf, index::Int, values) 
+ insert!(listStore::GtkListStoreLeaf, iter::TRI, values) 
empty!(listStore::GtkListStore)

delete!(listStore::GtkListStore, iter::TRI)
+ deleteat!(store, iter::TRI)
-splice!(listStore::GtkListStoreLeaf, index::Int) 
+ deleteat!(listStore::GtkListStoreLeaf, index::Int) 
+ pop! = deleteat!(store, length(store))


length(listStore::GtkListStore)
+ size(listStore) = (length(listStore, Gtk.ncolumns(GtkTreeModel(listStore))))


isvalid(listStore::GtkListStore, iter::TRI)
```
## indexing
```
+ getindex(listStore::GtkListStore, iter::TRI, column::Integer)) = getindex(GtkTreeModel(listStore), iter, column)
+ getindex(listStore::GtkListStore, iter::TRI) = getindex(GtkTreeModel(listStore), iter)
+ getindex(listStore, row::int, column) = getindex(listStore, iter_from_index(listStore, row), column)
+ getindex(listStore, row::int) = getindex(listStore, iter_from_index(listStore, row))
```
## what of setindex! ??

## no symmetry here, should this be addressed?
## get store from a view
```
getproperty(view, :model, Gtk.GtkListStore)
```
## set store for view, setproperty! fails 
```
Gtk.G_.model(view, store)
```

## Tree store
```
push!(treeStore::GtkTreeStore, values::Tuple, parent=nothing)
unshift!(treeStore::GtkTreeStore, values::Tuple, parent=nothing)
insert!(treeStore::GtkTreeStoreLeaf, index::Vector{Int}, values; how::Symbol=:parent, where::Symbol=:after)

delete!(treeStore::GtkTreeStore, iter::TRI)
deleteat!(treeStore::GtkTreeStore, iter::TRI) = delete!(treeStore::GtkTreeStore, iter::TRI))
empty!(treeStore::GtkTreeStore)
+ deleteat!(treeStore::GtkTreeStoreLeaf, iter::TRI) ## delete at
-splice!(treeStore::GtkTreeStoreLeaf, index::Vector{Int}) ## delete at as new name
+deleteat!(treeStore::GtkTreeStoreLeaf, index::Vector{Int}) 


isvalid(treeStore::GtkTreeStore, iter::TRI)
isancestor(treeStore::GtkTreeStore, iter::TRI, descendant::TRI)
depth(treeStore::GtkTreeStore, iter::TRI)

getindex(treeModel::GtkTreeModel, iter::TRI, column::Integer)
getindex(treeModel::GtkTreeModel, iter::TRI)

setindex!(treeModel::GtkTreeModel, value, iter::TRI, column::Integer)
setindex!(treeModel::GtkTreeModel, values, iter::TRI)
```


## Selection
```
- selected(GtkTreeSelection): returned (model, iter) but with error
+ selected(GtkTreeSelection): return GtkTreeIter or error if no selection
select!(GtkTreeSelection, Iter)
+ selected_rows(GtkTreeSelection) return GtkTreeIter[] or error if no selection
selectall!(GtkTreeSelection)

unselect!(selection::GtkTreeSelection, iter::TRI
unselectall!(GtkTreeSelection)
```