[doc parser] Allow default values for text entries.

When a ValueList node contains a single element,
instead of becoming a combobox with one choice,
it now becomes a default value for a text/path entry widget.
[exporter] Prepare for allowing partial exports.
[wip] added a default snippets xml file
implemented ranking algorithm

Loading of snippets is much slower (~10s for 150k snippets)...
Gotta look for a way to load them faster.
first implementation of sorting

Problem: loading / searching becomes really slow (around 300%) because sorting and searching are handled by the GtkTreeView.
We must implement paging and sorting on the sqlite layer.
added indexes for fast sorting
created database submodule
implemented database pager
updated todo
Merge branch 'feature/snippets-sorting' into develop