Implement the mulibuffer point layer generator widget

[mvmaltitz]

We need a widget that contains these elements:

• [ ] A text description explaining the input data.
• [ ] A QgsMapLayerComboBox to select the point layer
• [ ] A radio button that has the Travel Mode. Walking should be selected as a default
• [ ] A radio button that has the Measurement. Distance should be selected as a default
• [ ] A text box to enter the buffer increments. The default should be 250, 500, 750, 1000, 1250

Input:

• "Default Multi Buffer Distances": "250, 500, 750, 1000, 1250",
• "Default Multi Buffer Travel Mode": "Walking" (with Walking, Driving being valid options)
• "Default Multi Buffer Measurement": "Distance" (with Distance, Time being valid options)

This should be used to define the initial distances

Output:

A dictionary of item values for the Layer Node. From this dictionary the following elements should be populated by this widget:

• "Multi Buffer Distances": "0,0,0",
• "Multi Buffer Travel Mode": "Walking" (with Walking, Driving being valid options)
• "Multi Buffer Measurement": "Distance" (with Distance, Time being valid options)

These entries should be appended to or overwrite entries with the same name as provided in the input dictionary.

See also #261 for the backend function

[hennie-k]

The "Multi Buffer Travel Mode" and "Multi Buffer Measurement" inputs are not currently included in the input dict and needs to be included for this widget to be completed