Fix 'weighted mean' aggregate support in <perspective-viewer>

[texodus]

New implementations:

• Reimplements weighted mean" aggregate type in new ` UI
• Reimplements delete() for <perspective-viewer>

Fixes:

• Fixes default column selection for Chart plugins with minimum column requirements, such as 'X/Y Scatter'. When applied to a dataset with zero or too-few numeric columns, this logic would fail and select all columns instead, rather than choosing a minimum of non-numeric columns as a second choice.
• Fixes date/datetime filter validation, which would previously not allow invalid date or datetimes to be typed, like 1/1/0201, even if they were intermediate states while typing a full date like 1/1/2016.
• Fixes editable support in perspective-workspace, which while no longer a config variable for <perspective-viewer>, will remain in <perspective-workspace> for the time being.
• Fixes open_table() client method in JavaScript to return a Promise, making it consistent with other proxy-returning functions in the perspective API.
• Fixes filter and various other top-panel controls to pull theme variables rather than hardcode colors.
• Fixes bug which caused column selector to not re-render when the column list does not update, e.g. when changing from 'X Bar" to "Datagrid", which left the top column selector greyed-out until render was forced.
• Fixes columns so they are removed from the active column list when drag/dropped to a pivot position.
• Fixes CSS dimensional properties like position to be consistent and simplified between themes.
• Fixes lint to apply to examples and cpp, and auto-applies fixes as "no author".
• Fixes a bug which caused the plugin-config dropdown to not update after a call to restore() (though the plugin itself does switch and render correctly).
• Fixes theme variables which apply to modal to be properly instantiated, e.g. for moncao.
• Fixes <perspective-workspace> persistence for attribute-preserved settings like selectable.
• Fixes filter dropdowns to consistently open targetting the correct filter, which would drift when the settings control was toggled.
• Fixes perspective-webpack-plugin to correctly outline dynamic imports when inline: false.