Add showClear option to Calendar component

[Robin-front]

Calendar already implement this feature, but did't export it.
Because inner showClear prop is true by default, so I set default value to true.

Please note RangeCalendar also has showClear and default value is false.

trace: #345

[codecov-io]

Codecov Report

Merging #353 into master will decrease coverage by 5.93%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #353      +/-   ##
=========================================
- Coverage   94.43%   88.5%   -5.94%     
=========================================
  Files          63       9      -54     
  Lines        1958     600    -1358     
  Branches      338     158     -180     
=========================================
- Hits         1849     531    -1318     
+ Misses         96      58      -38     
+ Partials       13      11       -2

Impacted Files	Coverage Δ
src/date/DateInput.js	83.33% <0%> (-3.51%)	⬇️
src/mixin/CommonMixin.js	87.5% <0%> (-3.13%)	⬇️
src/month/MonthTable.js	89.06% <0%> (-2.71%)	⬇️
src/RangeCalendar.js	87.5% <0%> (-2.04%)	⬇️
src/mixin/CalendarMixin.js	82.14% <0%> (-1.47%)	⬇️
src/index.js	100% <0%> (ø)	⬆️
src/util/index.js	100% <0%> (ø)	⬆️
src/picker/placements.js	100% <0%> (ø)	⬆️
src/range-calendar/CalendarPart.js	100% <0%> (ø)	⬆️
src/year/YearPanel.jsx
... and 52 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6e3010d...b956360. Read the comment docs.