Adds support for -> operator, which allows to wait for various events

Adds support for -> operator, which allows to wait for various events

Consider following situation (in batch mode): first you TAP on icon on
home screen and launch an application, then you want to TAP on some
icon inside the application. Now you've to wait for the application to
initialize and appear foreground. Operator -> is to solve this issue:

TAP('Phone') -> gui()

means execute left side (TAP), then wait for conditions on right side
(gui - context must change, new app appear and so on). Currently only
two conditions are supported - gui (new context, new window and so on)
and tts (screen reader did read something).

Change-Id: I72d00335c360acf97979825b73ed0c98040a0dc4