[FIX] chart: tooltip issues

[hokolomopo]

Description:

[FIX] chart: handle tooltip with colons in the label

To split the labels and values in the tooltips, we did a simple split
by colon, since our labels had the format label: value. But this
obviously breaks when the label itself contains a colon.

With this commit, we now use the beforeLabel tooltip callback to
get the label, and the label callback to get the value instead of
doing everything in the label callback.

[FIX] chart: remove empty title in tooltip

If there is no title in the tooltip, we still displayed the div
containing the title. That led to a small empty space at the top
of the tooltip.

Task: 4586683

review checklist

• feature is organized in plugin, or UI components
• support of duplicate sheet (deep copy)
• in model/core: ranges are Range object, and can be adapted (adaptRanges)
• in model/UI: ranges are strings (to show the user)
• undo-able commands (uses this.history.update)
• multiuser-able commands (has inverse commands and transformations where needed)
• new/updated/removed commands are documented
• exportable in excel
• translations (_t("qmsdf %s", abc))
• unit tested
• clean commented code
• track breaking changes
• doc is rebuild (npm run doc)
• status is correct in Odoo

[robodoo]