En route to on-canvas gradient editing, add support for persistent
handle selection to GimpToolLine (a handle being either an endpoint
or a slider).  Handles are selected through clicking, however,
unlike before, the selection persists after the mouse is released.
A new "selection" property specifies the currently-selected handle
(who knows, maybe in the future we'll add multi-selection), and a
new "selection-changed" signal is emitted when the selection changes.

The visual feedback has been changed to better suit the new behavior,
and the behaviors yet to be added:  The selected handle is marked
using highlighting; the highlighting doesn't change while hovering
over other handles.  Only the hit-test circle is used as hover
indication, however, we use a fixed-size circle, and only show the
circle for the currently hovered-over handle -- no more trippy
expanding circles :)

A few minor changes along the way:

  - The selected handle is now the (first) one that's closest to the
    cursor, instead of the first one to pass hit-testing.

  - We don't move the selectd handle upon button-press, only upon
    motion, so that handles can be selected without moving them.

  - Show a MOVE cursor modifier when hovering over a handle.