System dependency checking needs a rework

[mgalgs]

Our system dependency pop-up dialog (smDialog) was using some private APIs which are being removed in Gnome Shell 46. Worse yet, the dependency checks have actually been hard-coded out (we always assume the system dependencies are installed) since the port to Gnome Shell 45 (f838b9786: "extension: Update for Gnome Shell 45"). The reason for hard-coding out the checks was because the old dependency check method relied on a try/catch of the library imports, which isn't supported in es6, and we felt like it was more important to get GS45 fixed than it was to get the dependency check fixed. Textbook example of technical debt.

We should bring back this check using modern methods (something other than an import try/catch) for the best possible user experience.

[mgalgs]

The old dialog was removed in 461475bea