Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

include/SDL3/SDL_power.h

@@ -0,0 +1,88 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SDL_power_h_
+#define SDL_power_h_
+
+/**
+ *  \file SDL_power.h
+ *
+ *  Header for the SDL power management routines.
+ */
+
+#include <SDL3/SDL_stdinc.h>
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ *  The basic state for the system's power supply.
+ */
+typedef enum
+{
+    SDL_POWERSTATE_UNKNOWN,      /**< cannot determine power status */
+    SDL_POWERSTATE_ON_BATTERY,   /**< Not plugged in, running on the battery */
+    SDL_POWERSTATE_NO_BATTERY,   /**< Plugged in, no battery available */
+    SDL_POWERSTATE_CHARGING,     /**< Plugged in, charging battery */
+    SDL_POWERSTATE_CHARGED       /**< Plugged in, battery charged */
+} SDL_PowerState;
+
+
+/**
+ * Get the current power supply details.
+ *
+ * You should never take a battery status as absolute truth. Batteries
+ * (especially failing batteries) are delicate hardware, and the values
+ * reported here are best estimates based on what that hardware reports. It's
+ * not uncommon for older batteries to lose stored power much faster than it
+ * reports, or completely drain when reporting it has 20 percent left, etc.
+ *
+ * Battery status can change at any time; if you are concerned with power
+ * state, you should call this function frequently, and perhaps ignore changes
+ * until they seem to be stable for a few seconds.
+ *
+ * It's possible a platform can only report battery percentage or time left
+ * but not both.
+ *
+ * \param secs seconds of battery life left, you can pass a NULL here if you
+ *             don't care, will return -1 if we can't determine a value, or
+ *             we're not running on a battery
+ * \param pct percentage of battery life left, between 0 and 100, you can pass
+ *            a NULL here if you don't care, will return -1 if we can't
+ *            determine a value, or we're not running on a battery
+ * \returns an SDL_PowerState enum representing the current battery state.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC SDL_PowerState SDLCALL SDL_GetPowerInfo(int *secs, int *pct);
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_power_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */