Add a hover pre-light effect to GtkTreeView and Nemo sidepane.

GtkTreeViews tended to feel unfriendly and static in Mint-X. This was
most noticeable in Nemo.

This modification adds a simple highlight to all GTK3 GtkTreeViews,
making them feel much more lively and responsive.

A separate sidepane modification is included for Nemo specifically.
Merge pull request #23 from wrouesnel/nemo-prelight-effects

Add a hover pre-light effect to GtkTreeView and Nemo sidepane.