Bug fixes, typos and code simplify

[GermanAizek]

@gammasoft71 I'm waiting code review. Thx

[gammasoft71]

First, thank you for your contribution.

List where your correction is not appropriate

• wx_up_down_domain_h : GetLabel() must be return GetValue().

• native::month_calendar.cpp, month_calendar.h and month_calendar.cpp : the selection_range method must have date_start and date_stop parameters even if wxWidgets does not handle them correctly. Future toolkits will do this.

• splitter.h my test is bad but your test is bad too. The dock param must be left, right, top or bottom.

For the other fixes, everything is good, thank's.

Remarks

For your next pull requests, can you divide your corrections into several commits (one per correction). This will make it easier to accept or reject requests.