Liveviever Workflow improvements

[Wikunia]

I'll use this issue for several workflow improvements that we might want to split into several issues in the future.
For now I think they make sense together.

Is your feature request related to a problem? Please explain.
Creating a longer video with Javis is not the most easy process for a variety of different reasons. One of them is long waiting times to render the video. For this reason I think #342 makes sense to render only part of the frames but it isn't that fast either as
previous frames need to be processed in some cases.

Currently the liveviewer is closest to the thing I envision for the future of an easy way to preview parts of a long video.

The current problems with it are:

• The size of the viewer is bigger than the video size due to the slider. When one wants to create 1080p videos on a 1080p monitor it basically can't be used
• Whenever one changes the Julia script and reruns it, a new liveviewer window opens
• It's nice for checking if stuff is at the right place on the image but one loses a bit the animated aspect when there is no possibility to play the animation.

Describe the solution you'd like
Possible solutions for the above problems are:

• One should be able to set the size of the liveviewer window and down scale the image to fit it
  • It would be even more awesome if one could rescale the window
• I would like to just update the image in the current gtk viewer. Furthermore it should not reset the frame such that one can basically change some non animating part and test different positioning very easy. Currently I sometimes copy that code over and run it with Luxor to check whether it looks good. Would love to avoid that 😄
• Maybe we can add an extra text box where one can specify a framerate and a play button. For this it would possibly make sense to save the single frames to memory or file to be able to display them faster without a need to recompute them.