Improve the add_straight_line module

Currently the module for arbitrary lines takes to much space and is not user friendly.

My proposition is to reduce the space of the module and to have it more robust. Requirements of the implementation:
- The single arbitrary line attributes should be as small as possible to not take to much space.
- One should be able to add, modify and remove arbitrary lines one by one
- Each line added is shown in the encoding panel as a small bullet

# proposition 1 - popup window
To add the new line user clicks (+) button which triggers small window to pop, with inputs for value, color and label. After submitting small bullet describing the line stays in the encoding panel and new straight line is drawn on the plot. One can also remove (+) and modify (~) the line by clicking relevant buttons.


# proposition 2 - collapsible div
To add the new line user clicks (+) which adds collapsible-div with inputs for value, color and label. In contrast to the option-1 these inputs does not disappear but they can be hidden by folding the div to the minimal size. Modifying the lines would require just unfolding div and clicking the inputs as usual. User can remove single line inputs by clicking (x)


Design file
https://app.diagrams.net/#G12KVOtFwNMMgFsrjpRAgLtHpOr5wSd4JN