One small stabilization fix.

Constants from CommandCustomizationSupport are used instead of
CommandExecutionSupport.

Variables and Commands test suites added to both validation and full test bags.

Initial revision of commands testing suite. The validation part of suite is complete.

Initial revision of variables testing suite. The suite is complete.

* Add "Show" filter in the Suggestions view, which lets you
easily filter by suggestion type / category.

* http://www.netbeans.org/issues/show_bug.cgi?id=28304
Improve the directory-task-scanner

Removed the unconditional addition of the "Scan Tasks From
Files..." action from the Tools menu; added it to the
Tools infrastructure such that it shows up in the Tools
menu for directories in the explorer; also, if a directory
is selected it shows up in the main Tools menu. There
is no longer a dialog; the recursive option doesn't seem
necessary (when do you ever want it?) and for the skip-
non-comments option I use the current system option value.

* Add SuggestionManager.remove(List) for performance reasons
(and for symmetry with add(List), which is also needed to
preserve order among a set of registered tasks.)
Add SuggestionProvider.getTypes() which identifies the
types of tasks added by a provider.

* Remove the "Check Copyright" option from the docscanning
module since it's no longer part of the editor view; it's
using the Suggestions framework and will use its generic
enable/disable scheme.

The error reporting in CommandProcessor is improved.
When creation of a customizer or command execution throws some
exception, the user is properly notified about it.
Two null pointer exceptions are fixed in ExecuteCommand and UserCommandSupport.
This is a fix of issue #28508.

custom solution for "select another node after deletion" was
replaced with explorer's one (see #13348)

Redundant unused messages removed.

Better recognize, whether the refreshed file is a folder or not.
This is a fix of issue #28925.