More consistent naming conventions for choice control items...

... This commit leaves at least a comment for all choice controls of all
built-in effects and commands.

Use CamelCase with no spaces for the internal identifiers, given in
wxT("...")

Use more idiomatic English with spaces in the user-visible names, given in
XO("..."); these strings go to the message catalog for internationalization

Do not use underscores

In case these strings are the same, give it just once in XO("..."), but if in
a future version it is decided to change the name in XO("..."), then the
original should be kept also in a distinct wxT("...") string, so that
saved configuration or plugin settings or macro files don't become gratuitously
incompatible just because of that user-visible name change.

Left TruncateSilence alone for now -- I think that choice demands a rewrite
as a slider instead.

src/commands/SelectCommand.cpp

@@ -46,12 +46,12 @@ explicitly code all three.
 const int nRelativeTos =6;
 static const IdentInterfaceSymbol kRelativeTo[nRelativeTos] =
 {
-   { XO("Project Start") },
+   { wxT("ProjectStart"), XO("Project Start") },
    { XO("Project") },
-   { XO("Project End") },
-   { XO("Selection Start") },
+   { wxT("ProjectEnd"), XO("Project End") },
+   { wxT("SelectionStart"), XO("Selection Start") },
    { XO("Selection") },
-   { XO("Selection End") }
+   { wxT("SelectionEnd"), XO("Selection End") }
 };
 
 bool SelectTimeCommand::DefineParams( ShuttleParams & S ){
@@ -169,6 +169,8 @@ bool SelectFrequenciesCommand::Apply(const CommandContext & context){
 const int nModes =3;
 static const IdentInterfaceSymbol kModes[nModes] =
 {
+   // These are acceptable dual purpose internal/visible names
+
    /* i18n-hint verb, imperative */
    { XO("Set") },
    { XO("Add") },