Menu bar - added mnemonics to more menu items

[ham1]

Description

• added mnemonics to more menu items
• general refactor/tidy of code

Motivation and Context

To make navigation easier, e.g. now to open most recent project, Alt+F, Alt+E, Enter.

How Has This Been Tested?

Manually on Linux Mint

Screenshots (if appropriate):

Types of changes

• Dev improvement and
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

[codecov-io]

Codecov Report

Merging #352 into trunk will increase coverage by <.01%.
The diff coverage is 84.93%.

@@             Coverage Diff              @@
##              trunk     #352      +/-   ##
============================================
+ Coverage     58.28%   58.28%   +<.01%     
- Complexity    10217    10218       +1     
============================================
  Files          1160     1160              
  Lines         74426    74408      -18     
  Branches       7343     7337       -6     
============================================
- Hits          43378    43369       -9     
+ Misses        28552    28546       -6     
+ Partials       2496     2493       -3

Impacted Files	Coverage Δ	Complexity Δ
...c/core/org/apache/jmeter/gui/util/MenuFactory.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...core/org/apache/jmeter/gui/util/JMeterMenuBar.java	76.21% <87.32%> (+0.56%)	49 <7> (+1)	⬆️
...apache/jmeter/extractor/TestBoundaryExtractor.java	97.88% <0%> (ø)	17% <0%> (ø)	⬇️

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