[16.0][FIX] web_timeline: Redraw issues with initial mode

[houzefa-abba]

This commit fixes redraw issues when setting a mode attribute in the timeline view tag.

This mode specifies a default scale one would want to set; same as when clicking on Day/Week/Month buttons at the top of the view.

Initial rendering had issues here because data was loaded too soon, before the timeline component was rendered/ready. The fix is to load data into the component only after initial redraw event, called changed (see https://visjs.github.io/vis-timeline/docs/timeline/#Events).

There was old code attempting to call on_scale_xxx_clicked methods at load time to simulate clicks on these Day/Week/Month buttons, but these methods have been renamed so this code is no longer working.

This commit also removes the current_window instance variable, not needed and actually confusing as the timeline component already maintains its own start/end information (which we can query with timeline.getWindow()).

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Screenshots below with project_timeline task view with mode="month"

Before

Only 1 line at the bottom, rest is missing

After