WT: allow copying current frame if no selection is found

[Joey79100]

WadTool's animation editor allows you to copy and paste a selection of frames of an animation.

However, it requires that you first select the frame(s) you want to copy, and then you can copy.

Considering you always have 1 frame hovered in the timeline (maybe except when there's no animation? very specific case), WadTool should allow copying the current frame if the user has done no selection.