Use the `:parts` and `:theme` system for all existing components

Newer re-com components (nested-grid, tree-select, dropdown) can use a :theme prop. :theme expresses an update to a component's props (e.g. style & class), as a function of the current props and a context. The context includes :part and :re-com. :part is a qualified id. :re-com is a map describing global configuration (such as color palettes) and any dynamic state (such as {:drop-down :open}).

As a user, you can write a single :theme function and "register" it once, to apply it to every re-com component. This theme-function can vary its behavior based on the :part (for instance, with a multimethod), gathering together all your visual adjustments for the app.

You can also pass a :theme function to an individual component, conveniently extracting the logic of styling from that component's implementation.

Complementing :theme, I've also expanded the :parts system. Previously, a :parts value could only be a map. Now, newer re-com components support strings, hiccups and functions. Strings & hiccups will replace the entire part, while a function will replace its component function.

The way :theme and :parts works is mostly stable in these newer components, so it would be great to add this feature to all our current components.

• alert
• box
• buttons
• checkbox
• close-button
• datepicker
• dropdown
• input-text
• typeahead
• input-time
• splits
• modal-panel
• error-modal
• multi-select
• popover
• progress-bar
• radio-button
• selection-list
• slider
• horizontal-tabs
• bar-tabs
• pill-tabs
• table-filter
• tag-dropdown
• text
• throbber
• tour
• tree-select
• v-table
• simple-v-table
• nested-grid