Allows selecting individual widgets in Gtk CSS using `#name` selector.

For example the snippet [from my comment](https://github.com/geany/geany/issues/2468#issuecomment-610695277) on #2468 could be made to apply to just the document notebook like:

```css notebook#notebook1 > header.top > tabs > tab { padding: 0px 12px 0px 12px; } ```

Tested with Gtk Inspector. You can view, comment on, or merge this pull request online at:

https://github.com/geany/geany/pull/2469

-- Commit Summary --

* Set a default widget name on all GtkBuilder widgets.

-- File Changes --

M src/ui_utils.c (5)

-- Patch Links --

https://github.com/geany/geany/pull/2469.patch https://github.com/geany/geany/pull/2469.diff