* Reworking notebook action bar functionality and appearance.
* added separator
* Revised notebookActions for collapse, clear and trusted such that they can be implemented with a boolean set to T of F and show labels or shift them into tooltip for accessibility. Updated styles for select boxes. Added toolbar icons to common icon location. Split icon definition for use as masked or background-image.
* Completed styles for action icons: collapse, clear and trusted. Added theme colors. Simplified icon behavior styles.
* Made maskedIconClass optional. Added theme colors for toolbar icons, select box border and dropdown arrow. Experimenting with adding masked icon to pseudo element so I can pull out label text from icons.
* Added icons styles to handle masked SVG elements as pseudo element beside button text. Added icons using this method to respect the color theming system.
* Adjusted styles for the cell and run all icons in notebook toolbar.
* Prepped notebook toolbar with placeholder icon for Underline action. Implemented Underline action. Added custom --wip-- ButtonMenu control, a modified copy of DropDown.
* Revised colorRegistry and corresponding notebook styles. Removed unused code from new custom control: buttonMneu. Revised icon styles to create a dropdown arrow for buttonMenu.
* Added new icon for Underline action.
* Removed comment from needed markup.
* Replaced actionItemProvider with optional undefined per DropdownMenuActionViewItem constructor.
* Cleaned up new control, removing unneeded code and referencing what the class needs. Corrected style declaration for overriding input box padding. Removed unused notebook color styles. Scoped element styles to the toolbar so others outside the toolbar are not affected.
* Removed unnecessary !important from style override.
* Removed reference to unused color entry.
* Syntax cleanup.
* Put notebook toolbar improvements behind the preview flag. This involves some conditionals and CSS classes.
* Updated icon used for Manage Packages. Created and updated styles for notebook toolbar icon spacing. Modified notebook.component contributed actions so that the label text is shifted into the title attribute. Added new icon for Not Trusted toggle.
* Replaced SVG code for not-trusted icon.
* Addressed PR feedback: changed masked classname. Revised component and CSS accordingly. Removed unnecessary instance of in-preview class. Fixed code logic that assigns label text to tooltip on incoming contributed action
