elm_calendar: provides way for configuration of weekend text color

elm_calendar: provides way for configuration of weekend text color

Summary:
Suggest new mark_type for elm_calendar_mark_add().
The mark names are "weekend/day1", "weekend/day2".
It can be used for each country.
Some country can apply to "weekend/day2" to friday if day rest only a day for a week.
Until now, there is no way to separate holiday, saturday and sunday.

Test Plan:
1. saturday
```
struct tm t = { 0, 0, 12, 6, 0, 0, 6, 6, -1 };
elm_calendar_mark_add(obj, "weekend/day1", &t, ELM_CALENDAR_WEEKLY);
```
2. sunday
```
 struct tm t = { 0, 0, 12, 7, 0, 0, 0, 0, -1 };
 elm_calendar_mark_add(obj, "weekend/day2", &t, ELM_CALENDAR_WEEKLY);
```

Reviewers: CHAN, cedric, raster

Reviewed By: raster

Subscribers: raster, akanad, id213sin, cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4148