Do not make a new keyframe if double clicking on an existing keyframe

[khoidauminh]

Based on the message from the Software's discord server, quoted:

Not sure if it's been discussed here but: First, double clicking in anywhere in the timeline gives you a new keyframe. Double clicking on a keyframe appends one after it. When you need to move a keyframe, you need to click on it first before you can drag it around. If I happen to do this too fast, pencil2d registers this as a double click and does the keyframe appending. This can be annoying

I think it'll help a lot if pencil2d distinguishes between double clicking on an empty space and on a keyframe

This PR does exactly what the message above wants to. When double clicking on an existing keyframe, the program won't make a new keyframe, instead only selects it. Beneficial for those who want to drag the keyframe around in a quick way.