Minor cleanup in the gmail plugin, to be clear about what re're returning in the
response.
Added some documentation about the plugins in the readme.
Fix issues with getSelectedWord, when the last character of the selection was a
space, and the word returned was '', by using trim() on the selected string.
Fix a long-stading issue when the autocomplete dialog was triggered with an
automatic first word selection, without manually-selected text.
The issue was caused by the fact that after triggering the dialog.close method,
the focus doesn't return to the editor right away, but has a slight delay.
This caused the window.getSelection() method to return the selection from the
dialog search field, instead of the selection in the editor.
Create a general method for traversing the DOM in search for the next tabindex
item (for when moving to the next button with Tab, and tab is assigned as the
autocomplete shortcut), so we don't have to manually do this in each plugin.