fix(graphics): only show the button if the graphics can be switched

[wash2]

should fix #173. This makes the button press-able if the graphics can be switched.

[jacobgkau]

Confirmed this does make the button unclickable (which means it also doesn't highlight on mouse-over) on an integrated system. The icon and the word Integrated still show up by default. Not sure if @pop-os/ux would want to take a look at this to ensure it's not potentially confusing. (This would probably still be an improvement over the non-functional menu, even if it was confusing.)

[maria-komarova]

If there are no modes available to switch, then the applet shouldn't show on the panel. The applet should only be present for systems where the modes are available.

[wash2]

Ok this should exit now when the graphics are not switchable

[jacobgkau]

This now causes the button to not show up on an integrated system.

This currently does cause an extra bit of space to appear between the surrounding icons where the applet is positioned (possibly just 1px, looking at the code?) See below for a comparison of the Graphics applet being in two positions (open them in tabs and flip between them to see the difference):

The space is also possible to find by moving the mouse across all the icons and seeing where there's a larger gap between the hover states.

[wash2]

Hmm ok Maybe the panel is not updating the layout after it exits? I'll test it and maybe make a fix for the panel.

[wash2]

pop-os/cosmic-panel#79 should fix the gap. It wasn't removing the spacing for the removed applet after it exited.

[jacobgkau]

Confirmed that issue is fixed; on a non-switchable system, the panel now looks identical regardless of the graphics applet's position, and there's no extra space between icons where that applet is:

I've also confirmed the menu still shows up as expected on a system with switchable graphics.