Separate layer manager from toolbar (and other minor UI tweaks)

[naschmitz]

Changes:

• Separate LayerManager from Toolbar. There are several implications:
  • a new "top right layout box" was created to layout the widgets horizontally (we can generalize the logic later),
  • the container ipyleaflet.WidgetControl now has transparent_bg=True with the background set by the widget container to prevent a background over the entire "top right layout box", and
  • added a reverse header mode on the container for panels that open down and to the left.

• Keep the basemap selector resource select field visible when not needed, and fill with "---".

• Other UI tweaks, such as using the correct icon on the inspector and padding around the container header items.