Add "trim" attribute to "add" command

[FeodorFitsner]

trim allows to specify the maximum number of children to leave after adding new child controls.
Negative trim value trims items from the end; positive - from the start.

For example, if we have the following controls tree:

page
  stack id=lines
    text value='Line 1'
    text value='Line 2'
    text value='Line 3'

Running add to=lines trim=3 text value='Line 4' will result into:

page
  stack id=lines
    text value='Line 2'
    text value='Line 3'
    text value='Line 4'

so controls were trimmed from the start to have a total number of children at 3.

On the other hand, running add to=lines at=0 trim=-3 text value='Line 4' will result into:

page
  stack id=lines
    text value='Line 4'
    text value='Line 1'
    text value='Line 2'

so new control was insterted to lines stack at position 0 and controls were trimmed from the end to have a total number of children at 3.