eliminate images "jumping" effect

after loading the properties in image viewer
Show an icon in the Extension Manager whenever updates to installed
extensions are available: a red dot with counter overlaid on the Installed
tab's icon.
disallow tabbing to prev and next buttons
correction
Code review: add docs; tweak color per Larz; test
ExtensionManagerViewModel.notifyCount in a few unit tests
Merge pull request #5838 from adobe/pflynn/extman-update-notification

Show update notification icon on Extension Manager "Installed" tab
Merge pull request #5848 from adobe/randy/issue-5766

Disallow tabbing to prev and next buttons
Merge pull request #5867 from martinstarman/czech-lang-update6

Czech lang - correction
Merge pull request #5775 from Simek/master

eliminate images "jumping" effect after loading the properties in image viewer
Merge changes in master and resolve conflict in brackets.less.