[wishlist] make LPE add dialog easier to navigate by keyboard

[Liam P. White]

Ubuntu GNOME 14.04

The trunk Live Path Effect Add dialog is a little bit awkward to use solely with the keyboard.

The attached patch file helps to ease this by catching the "enter" keypress event before the treeview does, so that the selected path effect can be added without tabbing outside the treeview to select the button.

Additionally, the new code also catches the Escape key, allowing easy exit from the dialog compared to the two escapes previously necessary to defocus the treeview, then exit the dialog.

EDIT: made the treeview grab focus on showing of the dialog as well.

[ScislaC (scislac)]

Fix committed in 13305. Thanks Liam!