text: Title: fill_selection script not pasting 'hello world'
  Body: ## Classification:
  
  Bug
  
  ## Reproducibility:
  
  Always
  
  ## Version
  
  AutoKey version: 0.95.1
  
  Used GUI (Gtk, Qt, or both): Gtk
  
  Installed via: don't remember. Probably PPA.
  
  Linux Distribution: Ubuntu 18.04 
  
  ## Summary
  
  The fill_selection script from the API Examples does not paste 'HELLO WORLD', but the last clipboard item.
  
  ## Steps to Reproduce (if applicable)
  
  1. Create a new script with the contents of https://github.com/autokey/autokey/wiki/API-Examples#fill_selection-script
  ```
  import time
  cb = 'hello world'
  clipboard.fill_selection(cb.upper())
  time.sleep(0.2)
  keyboard.send_keys('<ctrl>+v')
  ```
  2. Set a desired abbreviation
  3. Set the checkbox "Trigger immediately"
  4. Save the script
  5. Trigger the script
  
  ## Expected Results
  
  display 'HELLO WORLD'
  
  ## Actual Results
  
  displayed was my previous saved clipboard text, that was not saved through AutoKey.
  
  Log:
  ```
  2019-02-02 10:50:25,257 DEBUG - service - Script runner executing: Script('ngg')
  2019-02-02 10:50:25,258 DEBUG - interface - Send special key: [<Key.BACKSPACE: '<backspace>'>]
  2019-02-02 10:50:25,258 DEBUG - service - Input stack at end of handle_keypress: deque([], maxlen=150)
  2019-02-02 10:50:25,258 ERROR - service - Ignored locking error in handle_keypress
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/autokey/service.py", line 206, in __tryReleaseLock
      self.configManager.lock.release()
  RuntimeError: release unlocked lock
  2019-02-02 10:50:25,259 DEBUG - interface - Send special key: [<Key.BACKSPACE: '<backspace>'>]
  2019-02-02 10:50:25,261 DEBUG - interface - Send special key: [<Key.BACKSPACE: '<backspace>'>]
  2019-02-02 10:50:25,262 DEBUG - interface - Send special key: [<Key.BACKSPACE: '<backspace>'>]
  2019-02-02 10:50:26,459 DEBUG - iomediator - Send via event interface
  2019-02-02 10:50:26,460 DEBUG - interface - Send modified key: modifiers: ['<ctrl>'] key: v
  
  ```
  
  ## Notes
  
  The [fill_clipboard script](https://github.com/autokey/autokey/wiki/API-Examples#fill_clipboard-script) works as expected.