Window state error improvements and argument rename

[eeter]

Changes (backwards incompatible):

• Error messages for keywords Window Should Be Maximized, Window Should Be Minimized and Window Should Be Restored changed to contain the actual state as Maximized, Minimized, or Restored instead of 0, 1, 2.
• Window Should Be Restored optional argument renamed to match the other state verification keywords: window_title -> locator

Closes #125

[rasjani]

Dunno if it makes any difference but i'd have kept the numeric output too just for scenarios where parsing the output and possible reacting into those would be a tiny tiny bit easier :)

[Kompakti]

🌻