Bug 357659

The user interface for the Bézier curve effect is broken in several ways:

1. Initially (and whenever returning to this effect) the top-right and bottom-left points are accessible, but the helper pointers are outside the effect's UI window.
2. When leaving the effect by selecting something else and on return all settings for the effect are lost. As soon as the user tries to edit the Bézier curve, all settings are reset.
3. Albeit the background coloring is enabled by default, only a black background in the effect's graph UI is shown. Toggling twice makes the color background appear, albeit always the first one. Changing the channel to work on does not change the UI coloring.

Note: I'm using locale DE_de, where the fraction separator is "," instead of ".".

Reproducible: Always

Steps to Reproduce:
1. Create Kdenlive project.
2. Add any clip to it.
3. Add Bézier Curve effect.
4. Try to change the handles.
5. Try to change the channel.

Actual Results:  
1. Handles are outside the UI graph region and thus inaccessible using the mouse.
2. UI graph background color is missing or does not change accordingly to channel selected.
3. Settings are lost when leaving the effect and later returning to it.

Expected Results:  
1. Initial all four handles should be accessible from the UI using the mouse.
2. UI should show colored background according to channel selected.
3. Settings must not be lost when leaving the effect and later returning to it.