PdeKeyListener.java: tab key behaves correctly if !editor.tabs.expand

If the `editor.tabs.expand` preference setting is set to `false` (tabs 
are *not* expanded to spaces), insert a tab character when the tab key 
is pressed. Previously nothing would be inserted.
Merge pull request #2607 from teddywing/insert-tab-char-on-tab-keypress-if-not-editor.tabs.expand

PdeKeyListener.java: tab key behaves correctly if !editor.tabs.expand