Added option to change object selection behavior

[bjorn]

Previously when clicking objects it would always take the top-most object from any visible and unlocked layer.

This new option allows changing that such that when available, an object from one of the currently selected layers is preferred. It can also be set to only do this, when the highlighting of current layer option is enabled (the new default).

This would close #354.

[bjorn]

I can't continue this patch for now, but a remaining change is to apply this new option also when group-selecting objects.

[bjorn]

Another option that may be good to add, is to only pick objects from selected layers. Then this change will resolve issue #752 as well.

[bjorn]

I'm a bit hesitant about all the modes this new option could have:

• Select from All Layers (current behavior)
• Select from Selected Layers
• Select from Highlighted Layers
• Prefer Selecting from Selected Layers
• Prefer Selecting from Highlighted Layers (suggested default)

Maybe @eishiya can help with considering which ones could be left out or how to represent it differently. For example, I could also imagine:

• Select from All Layers (current behavior)
• Select from Selected Layers
• Select from Highlighted Layers
  [ ] Select from All Layers As Fallback (checkbox to avoid too many options, maybe with better name)

And it could also be an option to:

• Prefer Selecting from Selected Layers, but in "Highlight Current Layer" mode, to select From Selected Layers Only

But how to represent that as an option? :S

[bjorn]

Maybe have two options:

Selection Behavior:

• Select from All Layers (current behavior and suggested default)
• Prefer Selected Layers
• Only Selected Layers

Selection Behavior when Highlight Current Layer is enabled: (better name?)

• Prefer Selected Layers (suggested default)
• Only Selected Layers

Or we could automatically raise the exclusiveness by one level in "Highlight Current Layer" mode.