Change static const to static constexpr wherever possible

Since we require C++11, most of these consts can be constexpr.

Two `static const ld` remain non-compile-time-evaluable because
they depend on the runtime `log` function. One `static const cld`
remains non-compile-time because `std::complex<T>` doesn't become
constexpr until C++14.

dialogs.cpp

@@ -15,7 +15,7 @@ EX namespace dialog {
 #if HDR
   #define IFM(x) (mousing?"":x)
 
-  static const int DONT_SHOW = 16;
+  static constexpr int DONT_SHOW = 16;
 
   enum tDialogItem {diTitle, diItem, diBreak, diHelp, diInfo, diIntSlider, diSlider, diBigItem, diKeyboard, diCustom, diColorItem, diListStart, diListEnd, diMatrixItem};