Remove UI subsystem

It was migrated to SciJava Common and ImageJ Common:
* https://github.com/scijava/scijava-common/commit/0d4b014dae36deae4a457a3ab3ed9595c6a3f601
* https://github.com/imagej/imagej-common/e55158394cbd5a7bd7ca73abf0bc2d43030ee671
Add service which checks for updates on startup

Previously, this was hard-coded into the UIService, which was the wrong
place for it. Now that the core UI subsystem has moved to SciJava
Common, let's take the opportunity to perform the update checking in a
better way: using a dedicated UpdateService. Over time, we can migrate
core updater functionality to this service, too.
Add service which displays the readme on startup

Previously, this was hard-coded into the UIService, which was the wrong
place for it. Now that the core UI subsystem has moved to SciJava
Common, let's take the opportunity to display the readme in a better
way: using a dedicated ReadmeService.
AboutImageJ: get base directory from App

This is more flexible than grabbing it via a static utility method.
Merge branch 'ui-split'

This branch eliminates the ij-ui component, migrating the
non-image-specific portion to SciJava Common, and the image-specific
part to ImageJ Common.

See:
https://github.com/scijava/scijava-common/commit/72370dfb6572b7e59c75227058672ba863061635
https://github.com/imagej/imagej-common/commit/ac4509c62550612e4e52f80bcc0078662524f428