Add App Engine flex new project wizard

[chanseokoh]

Fixes #2470.

This is a heads-up. It is based on #2545, so #2545 must go first. Also, I haven't added any tests. Changes in tes test files ATM are due to refactoring.

The wizard has a combo to select a sample template. Spring Boot is one of two choices.

Everything seems to work end to end, but I need to more extensive testing.

[codecov[bot]]

Codecov Report

Merging #2546 into chanseok-wip will increase coverage by 0.03%. The diff coverage is 62.92%.

@@                Coverage Diff                 @@
##             chanseok-wip    #2546      +/-   ##
==================================================
+ Coverage           72.63%   72.67%   +0.03%     
- Complexity           2381     2396      +15     
==================================================
  Files                 348      350       +2     
  Lines               13911    13963      +52     
  Branches             1389     1389              
==================================================
+ Hits                10104    10147      +43     
- Misses               3240     3250      +10     
+ Partials              567      566       -1

Impacted Files	Coverage Δ	Complexity Δ
...ct/standard/CreateAppEngineStandardWtpProject.java	100% <ø> (ø)	4 <0> (ø)	:arrow_down:
...newproject/flex/CreateAppEngineFlexWtpProject.java	97.72% <ø> (ø)	10 <0> (ø)	:arrow_down:
...ne/newproject/flex/AppEngineFlexWarWizardPage.java	50% <0%> (ø)	1 <0> (?)
...appengine/newproject/maven/MavenCoordinatesUi.java	92% <0%> (-1.88%)	14 <0> (ø)
...newproject/flex/CreateAppEngineFlexJarProject.java	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...e/appengine/newproject/AppEngineProjectConfig.java	97.56% <100%> (-2.44%)	22 <9> (+2)
...wproject/standard/AppEngineStandardWizardPage.java	96.55% <100%> (+0.12%)	7 <1> (+1)	:arrow_up:
...newproject/flex/AppEngineFlexWarProjectWizard.java	52.94% <100%> (ø)	3 <1> (?)
...oject/standard/AppEngineStandardProjectWizard.java	71.87% <100%> (ø)	5 <1> (ø)	:arrow_down:
...e/appengine/newproject/AppEngineProjectWizard.java	69.09% <100%> (ø)	7 <1> (ø)	:arrow_down:
... and 12 more

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