tidy: Fix some class member naming. (libmythui)

The clang-tidy "identifier naming" checker pointed out a number of
places where class member variables don't use the convention of
starting with "m_", or static variables don't begin with "s_".  Fix
these for naming consistency across the code base.  There are also a
handful of member variables names that were converted from the
"m_lower_case" form to the "m_camelBack" form.

https://clang.llvm.org/extra/clang-tidy/checks/readability-identifier-naming.html

mythtv/libs/libmythui/DisplayResX.cpp

@@ -20,15 +20,15 @@ static XRRScreenConfiguration *GetScreenConfig(MythXDisplay*& display);
 
 bool DisplayResX::IsAvailable(void)
 {
-    static bool checked = false;
-    static bool available = false;
-    if (!checked)
+    static bool s_checked = false;
+    static bool s_available = false;
+    if (!s_checked)
     {
-        checked = true;
+        s_checked = true;
         MythXDisplay display;
-        available = display.Open();
+        s_available = display.Open();
     }
-    return available;
+    return s_available;
 }
 
 DisplayResX::DisplayResX(void)