improve demand/select mode

[namdre]

• the combobox for selecting the type of demand object is huge
• the list in the combobox isn't sorted in any meaningful way
• some object categories could optionally be merged (and maybe we only want to show the list of merged types by default):
  • only "flow" instead of "flow route", "flow embedded route", "flow edges", ...
  • add a checkbox for "detailed list"
• selection via clicking (rework person selection in person plan mode #9606, Rework vehicle selection in stop plan mode #10702)

[namdre]

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