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.

[MrStevns]

Hi khoidauminh, thanks for the PR.

I implemented this feature with the goal that it should be easier to add addition frames to a Keyframe without having to move the existing ones on the timeline. From the thread though I realize that in its current state, the functionality is too easy to trigger.

Based on the feedback from Discord, I'll accept the PR for now.

[MrStevns]

A made a small change to account for soundclip length and also put the scrub call inside the branch.
Will merge PR now.