IJ1Helper: fix IJ1MenuWrapper#get

Corrected the implementation of the get method in IJ1MenuWrapper. Key
fixes include:

* Ignore trailing whitespace on menu names (as "Edit > ..." was being
  cut to "Edit " and not matching the "Edit" menu, for example).
* Test against existing menu labels and not names (IJ1 menus are named
  "menu1", "menu2", etc... but their labels, such as "File", "Edit",
  etc.. are what we want to compare.
* Added documentation to explain what this method is doing.

IJ2 menus should now be properly translated to IJ1 menu structures.
IJ1Helper: add javadoc for menu translation

Adds some basic javadoc for the methods used in translating IJ2 menus to
IJ1 menus.
DefaultLegacyService: remove unused import

Don't need the ShadowMenu import here any more.
IJ1Helper: also need to trim the leaf command

When adding the final leaf item to our menu structure, it was not being
trimmed of whitespace. This caused it to not match the name that was
registered as an IJ1 command and thus have no associated operation when
selected from the menu (and also causing 2 entries in the IJ1 plugin
list - one with whitespace, one without).

By trimming the actual command name, the menu item now matches the
command and executes properly when selected from the IJ1 menu.