Improve the appearance and usability of guide handles

This MR changes the behaviour and appearance of guide handles (the little dots that mark the origin point of a guide).

Summary of changes

• The size of the handles is now adjustable in the Preferences via the existing setting Interface → Handle size. Nonetheless, the guide handles react to this setting at half the rate of other handles; at large settings, the effective size of a guide handle is about half the size of an auto-smooth node. At the same time, a minimum size is enforced, equal to the previously hard-coded size. As a result of this mechanism, the handle is only enlarged when the Handle size preference is set to a value of 7 or larger. Since the default value is 3, the size of guide handles does not change on new installs or for users on default settings.

• The handle now has a hitbox and reacts to mouse events in the same way as the guide line itself. This makes it easier to interact with the guide. Taken together, these two changes address #3111 (closed).

• The colour of the handle is now the same as the colour of the guide, except when the guide is locked.

• When a guide is locked, its handle turns gray (and changes the shape to an "x", as before).

• The handle is always correctly centered on the guide line. This change also fixes a similar positioning error that affected node handles.

Fixes #2022 (closed); Fixes #2604; Fixes #3111 (closed);