New RangeSlider widget

[ppizarror]

• Added RangeSlider widget.
• Added KEY_TAB to controls.
• Improve tests and docs.
• Fix render after set value for drop select and toggle switch widgets.
• Widget set_position_relative_to_frame method now is private.

[codecov[bot]]

Codecov Report

Merging #353 (9306f24) into master (f1701aa) will increase coverage by 0.58%. The diff coverage is 99.54%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   96.27%   96.86%   +0.58%     
==========================================
  Files          60       61       +1     
  Lines       11812    12396     +584     
==========================================
+ Hits        11372    12007     +635     
+ Misses        440      389      -51     

Impacted Files	Coverage Δ
pygame_menu/widgets/__init__.py	100.00% <ø> (ø)
pygame_menu/widgets/selection/highlight.py	100.00% <ø> (ø)
pygame_menu/widgets/widget/menubar.py	99.20% <ø> (ø)
pygame_menu/utils.py	94.66% <93.47%> (-0.22%)	:arrow_down:
pygame_menu/_widgetmanager.py	99.62% <100.00%> (+0.01%)	:arrow_up:
pygame_menu/controls.py	100.00% <100.00%> (ø)
pygame_menu/examples/multi_input.py	98.92% <100.00%> (+0.04%)	:arrow_up:
pygame_menu/menu.py	95.02% <100.00%> (+0.04%)	:arrow_up:
pygame_menu/widgets/core/widget.py	97.66% <100.00%> (ø)
pygame_menu/widgets/widget/__init__.py	100.00% <100.00%> (ø)
... and 15 more

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