Multiframe Filters

This patch is for applying filters uniformly across each frame of an animated paint layer.

suppose you have an animated layer with a blue electric effect, for example. That layer could easily be made of dozens of unique keyframes. But now you decide that you'd rather have purple electric effect (or even one that shifts or flickers between colors over time), the easiest way to do that right now is to apply a HSV Adjustment filter across a single frame and then Ctrl+F to repeat the filter across every other frame in your animation. This is kind of a pet example, but you get the point.

The eventual solution would be to give animators the option of selecting a range of keyframes and allowing them to apply a single filter across each of the selected frames, using the active frame as a preview. Come to think of it, I'm not sure what a better default behavior is, filtering all frames or just one--but we will definitely make sure that animators can still do single frames. :)

This would be a step towards allowing for more multi-frame operations in general, which is something that bollebib has requested. https://bugs.kde.org/show_bug.cgi?id=373305 https://bugs.kde.org/show_bug.cgi?id=406327

How it works (so far):

1. Select a number of animation frames within the Animation Timeline Docker. (They don't have to be contiguous!)

2. Choose the filter that you want to apply from the Filter menu.

3. Set the filter parameters as you normally would, then check the Selected Frames checkbox at the bottom of the dialog, and press OK.

The active layer's active frame, as well as all of the other selected frames, will now have been filtered with the same parameters in a single undo-able action. This design should not interfere in any way with the filtering workflow of a single frame or non-animated layer!