GridConfig UX improvements

Desired behavior

Current GridConfig UI below.

Proposed enhancements:

• "Horizontal" should precede "Vertical" field, for better intuition
• "Position" field should be more clear which part of the grid it refers to, the center, a corner and which corner, etc.
• Consider making the "Color" group a linear layout as opposed to 2 x 2. Opposing argument: 2 x 2 saves space over vertical. They might fit horizontally.

Other enhancements:

• There has been community suggestion that we offer an interface in degrees for angles. Has not been discussed internally. If we make standard widgets to be reused across plugins, that might be something to add.
• If we do that, then similar could be said of colors in range 0-255, since color pickers online and in drawing tools usually give colors in that range.

Implementation suggestion

GridConfig plugin is located at https://github.com/ignitionrobotics/ign-gui/tree/ign-gui6/src/plugins/grid_config

Additional context

Original discussion here https://github.com/osrf/lrauv/pull/125