NF: Window startup messages interface

[mdcutone]

This PR adds setMessage and hideMessage attributes to Window. The application can set these messages to display informative text during startup to show resource loading and framerate testing are in progress.

Needs some additional work to set font sizes, position, etc. and possibly display graphics alongside the message. Might not work in VR as-is.

[codecov]

Codecov Report

Merging #5511 (5dff6a4) into dev (ec87ee9) will decrease coverage by 6.78%.
The diff coverage is n/a.

❗ Current head 5dff6a4 differs from pull request most recent head d561a1c. Consider uploading reports for the commit d561a1c to get more accurate results

@@            Coverage Diff             @@
##              dev    #5511      +/-   ##
==========================================
- Coverage   47.08%   40.30%   -6.78%     
==========================================
  Files          65       65              
  Lines        9749     9736      -13     
  Branches     1257     1255       -2     
==========================================
- Hits         4590     3924     -666     
- Misses       4905     5614     +709     
+ Partials      254      198      -56     

see 16 files with indirect coverage changes

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