Making .outputTooltip class on slider themable

[rorticus]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:
Making .outputTooltip class themable by removing it from the fixed CSS and adding it to the theme CSS.

Resolves #533

[dylans]

Do we need to also do something in @dojo/themes?

[codecov]

Codecov Report

Merging #540 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #540   +/-   ##
=======================================
  Coverage   97.31%   97.31%           
=======================================
  Files          32       32           
  Lines        2530     2530           
  Branches      642      642           
=======================================
  Hits         2462     2462           
  Misses          9        9           
  Partials       59       59

Impacted Files	Coverage Δ
src/slider/index.ts	98.33% <100%> (ø)	⬆️

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