Fix x-insert command

- now insert_text() and insert_snippet() will look for the front most text view before inserting
- if no text view was found a new document will be created before inserting
- for insert_snippet(): after insertion the key input focus will set to the found text view after insertion since a snippet needs user interaction afterwards
Use less private selectors

Those used are likely to change in the future.
Simplify view search code

We do a breadth first search starting with first responder and then content view so we still get the advantage of finding the view quickly but avoid duplicated code.
Added command 'filepanel'

- shows an Open File Panel or a Save File Panel
- returns a plist with keys 'clickedButton' and 'path' for one file or 'paths' for more than one file
- see help for option list
Add license.
Explicitly disable ARC
Add TMPlugInAPIVersion key to Info.plist