Add subset by polygon to mapApp()

[j-harbin]

Two known "bugs":

1. If in polygon mode, and user tries to zoom, the app will not zoom in (which is what we want) but it does cause an additional polygon point from the click that was required for the attempted zoom. This additional point can removed by simply clicking "u"
2. If the user subset by polygon and clicked "q" to indicate they are done, the app is NO LONGER is polygon mode. This means that the user IS ALLOWED to zoom with a brush. The zoom works correct, however, if the user clicks "u" to undo the zoom there will be an extra point in their Gray polygon. This again can be removed using "u".

I have tried many different ways to fix these, but I can't figure it out. They are not major issues as the user simply needs to click "u", however, it's worth noting.

[codecov[bot]]

Codecov Report

Merging #568 (e0eb70e) into develop (51f00c4) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #568   +/-   ##
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  Coverage    56.62%   56.62%           
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  Files           15       15           
  Lines         2024     2024           
========================================
  Hits          1146     1146           
  Misses         878      878           

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[dankelley]

Ta.