Theming fixes for disabled controls in Aristo2

These commits will change the theming of disabled controls in Aristo2. Rather than use dedicated images for the disabled state, this changes the theme to use the "regular" images and simply decrease their opacity via CSS.

Fixes #1788 

Edit: More information about these commits.

There are two methods used by the controls to control their disabled state. The first is via CSS opacity (checkboxes, radio controls, etc). The second is via dedicated disabled state images. This commit changes the first group to use CSS opacity exclusively. These controls use `<img />` elements to display the control.

However, there is no mechanism to set CSS opacity on members of the second group. I have opened up another issue (#1885) to consider adding CSS opacity controls to these groups.

Notable features of these commits:

* Default disabled opacity has been bumped up to 0.5 from 0.35 (4956218). Note that this only changes the opacity for controls in the first group mentioned above. This was because a value of 0.35 resulted in controls that were too light.
* Updated disabled controls for popup/pulldown/stepper/combo box/segmented/sliders. This includes a more refined disabled look that dims the outer border (closer to Cocoa behaviour).
* Updated placeholder text colours. The Cocoa placeholder text is the same colour as the disabled text. (bf29a4f)
* A small bug fix that removes a duplicated method in CPToolbar (6d843e6)