RFR: JMC-6368: Foldable threads table in the Threads page

[Alex Macdonald]

Hi,

The following webrev [0] addresses issue JMC-6368 [1], in which the user
should be able to fold away the table and graph on the Threads page to
reclaim screen real-estate.

I had previously attached two proof-of-concept gifs to the JIRA issue [1],
and after discussion on Slack it looked like taking the route of POC2
(adding toolbar actions along side existing page controls) was the short
term win, and that POC1 (UI components collapsing into a vertical bar of
some sort) should be investigated as it's own issue and potentially
implemented across the UI. This patch introduces two selectable items to
the toolbar which allow the user to expand either the table or graph to
fill the screen, and then fold away the other component. In a way to
prevent both components from being folded away (and viewing nothing) there
are controls to disable toolbar items when only 1 component is visible (one
is already folded), and enable them when both are visible (and one can be
folded away).

I've included a couple of screen shots that display the page with:
- both the table and graph visible [2]
- the table folded and the graph visible [3]
- the graph folded and the table visible [4]

and a couple of gifs to show
- the general flow of the controls [5]
- the folds persisting a reload of the jfr file [6]

I wrote a couple of uitests that verify some behaviour by interacting with
the toolbar items and verifying the weights of the SashForm; passing Travis
build available [7]

Thoughts?

Cheers,

Alex

[0] http://cr.openjdk.java.net/~aptmac/JMC-6368/webrev.00/
[1] https://bugs.openjdk.java.net/browse/JMC-6368
[2] https://imgur.com/XW99H5L
[3] https://imgur.com/d0d7Wln
[4] https://imgur.com/ibPgaWw
[5] https://imgur.com/QZzZZUN
[6] https://imgur.com/gUNfpTX
[7] https://travis-ci.org/aptmac/jmc-qa/builds/530418716