Fix disabled theme for Aristo2 checkboxes

Previously the Aristo2 checkboxes used dedicated images for disabled controls.

This commit removes the disabled controls and allows the disabled state to be set via CSS opacity on the "regular" checkbox controls.
Fix disabled theme for Aristo2 checkboxes

Previously the Aristo2 checkboxes used dedicated images for disabled controls.

This commit removes the disabled controls and allows the disabled state to be set via CSS opacity on the "regular" checkbox controls.
Merge branch 'fix-theme-disabled-controls' of github.com:ahankinson/cappuccino into fix-theme-disabled-controls
Fix disabled theme for Aristo2 radio buttons

Disabled state is now set via CSS opacity alone
Fix disabled theme for Aristo2 segmented controls

Disabled state is now set via CSS opacity alone
Fix CPComboBox disabled images

The new images are set at full opacity with the blue highlight colour removed
Fix CPPopupButton disabled images

The new images are set at full opacity with the blue highlight colour removed
Fix Pulldown Button disabled images

The new images are set at full opacity with the blue highlight colour removed
Fixed: Change default disabled opacity

Previously some disabled controls would be set with an opacity of 0.35, which resulted in controls that were visually too light.

This commit changes the default disabled opacity to 0.5.

Refs #1788
Formatting and whitespace
Updated combo box disabled images

These updated images have an outer stroke opacity of 20%, compared to 35% for the active images
Updated popup button disabled images

These updated images have an outer stroke opacity of 20%, compared to 35% for the active images
Updated pulldown button disabled images

These updated images have an outer stroke opacity of 20%, compared to 35% for the active images
Fixed: Combo box disabled state dims text

Previously, setting a CPComboBox to disabled would not dim the text. This commit fixes the theme state to include a setting for the text colour and text shadow colour.
Fixed: circular slider disabled image uses CSS opacity
Standardizing disabled text theming for popup and pulldown buttons
Defined file-level variables for text colors

This commit defines file-level variables for the disabled text colour, the disabled text shadow colour, and the placeholder text colour. It also adds or replaces these values in the controls where appropriate.
Fixed: Text shadow offset on enabled controls

Previously, the text shadow offset on a control that was active (e.g., a button that was set to default, or the active segment of a segmented control) would have a text shadow applied to it if the button was also disabled.

This commit negates the text-shadow offset so that the text appears correctly.
Reverse earlier commits that removed some images

The segmented control disabled state is not applied via CSS opacity, so the segmented control images must be present.
Slight tweak to the disabled slider knob graphic

The stroke opacity is reduced from 35% to 20% to bring it in line with the stroke opacity of other disabled controls