Fix the "smudgy" look of treeview elements

The treeview row-endings and expanders both have some problems with make them look dirty and smudged. The row-ending effect integrates poorly and lowers readability in most applications in GTK2 - removing it improves the overall appearance a lot:

Old:
![Old](http://i.imgur.com/7nCCrAb.png)

New:
![New](http://i.imgur.com/wWFmUKY.png)

The effect is more dramatic in other applications.

The treeview expanders looked very "smudgy" in the current theme due to
an imbalance of the grey colors around them. This commit removes the
imbalance and replaces it with clean transparencies and sharp lines,
greatly improving the overall appearance.

The expander icon size property is changed to match the new expander
icons in GTK2/3.

Old Look:
![Old](http://i.imgur.com/PJynM.png)

New Look:
![New](http://i.imgur.com/n1mZD.png)