Tidy up brush editor & fix exception

Newer Glade versions insist on writing internal child widgets for scale
buttons that older GTK and GtkBuilder versions don't support, even when the
Builder XML file's <requires/> field is set correctly.

Fix the resultant exception by manually deleting the plus_button and
minus_button internal children of the two (repeated, templated)
ScaleButtons we use.  We'll just have to be careful about doing that for a
few versions.

Fixes #46.

Also, a general and long-needed UI tidy-up for the brush editor:

* Symbolic icons for X-axis scale editors (still unintuitive though...)
* Put the scale editors at the outside & reduce margins a bit
* Use Gtk.ShadowType.IN for the main edit area - nicer for Ubuntu
* Silly padding widgets for where we had placeholders (Glade convenience)