Timing specification

Timing information can be specified in the following places:

Showing a screen for a given duration:
```
show Introduction for 10000
```
Displaying a component during the test run for a given duration:
```
cross() for 1000
```
Defining when the component should be shown. For this we use component timing expression which is specified before the component. For example:
```
at .+200 rectangle()
```
This is the general form of the timing expression:
```
at <target:><.><+ or -><miliseconds>
```
where:
- target: - is the name of the component this expression is relative to. If not given, timing expression is relative to the previous component.
- . means relative to the start of the target component. If not given time is relative to the end of the previous component.
- + or - - specified time is relative and is added or subtracted from the target time.
- <miliseconds> - a number of miliseconds
If timing expression is not given for the component, by default it is:
```
  at .
```
which means start at the same time as the previous component.

Examples:
```
  at 200 cross()           --  show cross 200ms after the phase begins (this is absolute time)
  at .-100 cross()         -- show cross 100ms before the start of the previous component
  at +100 cross()          -- show cross 100ms after the finish of the previous component
  at myrec:.+100 cross()   -- show cross 100ms after the start of the myrec component
  at myrec:+100 cross()    -- show cross 100ms after the finish of the myrec component
  at myrec:-100 cross()    -- show cross 100ms before the finish of the myrec component
```