MXRoyale layouts issues

This issue extends from a dev list discussion and relates to MXRoyale layouts emulation.

Attached are 3 file sets.

1. A Flex minimal reference example. This contains a single mxml applicaiton file and the results of a local build of that file (so it can be run without building if preferred)

2. An equivalent of the Flex example, built with the current (unchanged) MXRoyale lib.

3. An equivalent of the Flex example, built with a locally modified MXRoyale lib. The local MXRoyale modifications are also included in the zip file as the subset of files with modification (not a patch, full files). This means that it should be possible to overwrite these files into the MXRoyale lib and achieve the same results when building.

Quick explanation.
The example contains:
a. A minimal Grid/GridRow/GridItem test
b. A couple of Horizontal Layout tests (one of which is a solitary GridItem which is a horizontal container, the second one is a nested HBox setup with a sibling)

c. A very basic Grid layout without any complexity whatsoever.

The main tests are the first 2 which allow adding buttons programmatically inside their target containers which have percentage dimensions. These tests also allow removal of the buttons. They are just to observe the layout responsiveness within the containers and also at the parent level.

Zip Fileset 3 (up top) gets closest to the original Flex and I am using it so far without issue in some deeply nested layouts in a client app. But it certainly does not address all layout issues (there are still some layout issues in GridRow/GridItem which can be observed and which I intend to come back to, for example).

This issue is posted for review, consideration and feedback.
[FlexTest_Reference.zip](https://github.com/apache/royale-asjs/files/4735145/FlexTest_Reference.zip)
[royale_without_changes.zip](https://github.com/apache/royale-asjs/files/4735148/royale_without_changes.zip)
[royale_with_MXRoyale_changes.zip](https://github.com/apache/royale-asjs/files/4735149/royale_with_MXRoyale_changes.zip)

Start by comparing FlexTest_Reference.zip with royale_without_changes.zip and royale_with_MXRoyale_changes.zip, focusing on the Grid, GridRow, GridItem, and Horizontal Layout examples. Run the attached examples and inspect the modified files included in the third archive. Done would require a specific, agreed layout fix and confirmation that the responsive and nested layout behavior matches the reference.

Written by the indexing model from the issue text.